It's no less comfy than carrying a proper hiking one, although presumably it's not as good (or waterproof) for all day hiking.
Well I doubt comfort will be an issue. Whether you get a travel pack, hiking, or alpine pack, they're all comfy. THe difference is in the proportions of the pack's dimensions. I also mentioned the centre of gravity of travel packs are a bit far behind you, which means you may have to hunch forward a bit more if you're carrying lots of stuff.
The thing I've found really useful for travelling is that the rucksack straps zip away which is much better for lugging round airports and onto trains (and stops the straps being damaged by baggage handlers) and looks a bit more respectable if you need it to.
Yep, another benefit of the travel pack. They have airports in mind. Also, it's possible to put 2 or 3 locks on each pair of zippers to keep the pack entirely closed. This isn't really possible with hiking packs. People can reach into your pack, or even put something into your pack. This a particular problem in parts of Asia such as Indonesia.
You don't want to be the next Schappelle Corby, do you?
Lau said:
The other thing that is insanely useful is that it's got a long zip that lets you access everything easily – much better for city travelling than a hiking style one you have to access from the top (and stand there red-faced at check-in while you unpack your whole bag trying to find something

).
The other thing to consider is whether to get a top-loading or a front-loading pack. With a top-loader, you have to dig through the pack to find whatever you're looking for. With a front-loader, you unzip the front and see a cross section of everything in your pack, making it much easier to get things and put things where you want them to be.
Some hiking packs, such as my old, trusty Mountain Equipment Co-op (MEC) 60L hiking pack, has a vertical zipper so that you can easily get stuff from the middle of the pack. It was an awesome pack, but it didn't have enough space for winter clothes, which takes up far more space.
With my old 60L rucksack, if you tilt the entire pack onto its side and open the zip, it's just as good as a suitcase.

The Macpac Torlesse 75 hiking pack that I mentioned before has this. It's an awesome pack, but wasn't worth the extra £60 over my Caribee Leopard 70 L travel pack.
MEC sells awesome hiking packs at an awesome price. Too bad you're not in Canada.

My old hiking backpack was a MEC.
A zip-on day pack is sort of useful but not essential – they're not as comfortable as a decent day pack and when you're carrying it all together round unfamiliar cities, the daypack is really far away from you on your back and can easily be opened (or zipped right off the bag!) without you knowing because it's so far away behind you.
Yeah, my new pack had this, and while I was in Japan, I never used it.
I'm about the same height and weight as you and use a Lowe Alpine 70+20 which is probably a bit of overkill for what you want but packs like this have the big advantage of ajustable lumbar support (for instance I've short legs but a longish waist

) and for even walking short distances is a big plus.The heavier construction is good as being slung off and on planes takes it out of lightweight packs.Not so cheap but worth it,selection here:
http://www.foxsoutdoor.co.uk/rucksacks/mountain-packs/
Lowe Alpine stuff is awesome. Them and Macpac are probably upper tier, although the heavy construction generally means that you're always carrying an extra 0.5 - 1 kg.
EDIT: In terms of stuff for example I have a travel towel which is about as big as a novel, so I'm not going to need a ton of space, I'm also more than happy to use laundrettes regularly and intend to take 3 pairs of travel (thin) trousers and 2 pairs of jeans with me.
Five pairs of trousers? You're packing too much.
Jeans have an upside and downside. Jeans don't need to be washed. You can wear them 10-15 times and they're still OK. In fact, they become more comfy over time, and they're a tough material. However, they're not great in rain, they take forever to dry, they don't pack particularly small, and they're quite heavy. If you're going to bring jeans, just bring a pair you know you can wear out and look decent in. There may be an occasion where you may want to go out to a bar with new friends you meet, so they become handy in those situations.
1. A full water bottle
2. Liquid Pepto Bismal.
3. A roll of toilet paper, or a bunch of tissues for that purpose.
All good suggestions.
Depends on the seasons really and the exact places in those countries you want to visit.
If you come to South America on summer you will need little more than a bathing suit and something light as an external layer (think something like a GoreTex Paclite jacket).This will allow you to carry everything you need in a 70lt. pack. Personally I find it a very good size for carrying because it needs you to be as careful as possible with what you take, as every Kg. counts.
On the other hand, the mountains will require a lot more insulation (think Polartec, Windstopper, GoreTex XCR etc). For this you will need more space, I'd probably go for the 90lt.
This will also become more necessary the further you go south. For instance if you get further south in Chile than Puerto Montt or than Bariloche in Argentina you will start to need heavier clothing and you will encounter more rain. (Again, nothing too bad if you come in summer).
I probably wouldn't suggest he go for a 90 L backpack. It's too big, and if he packed well, he shouldn't need one. You can tell who's comfortable with travelling and who isn't by the size of their pack.

You don't need to bring everything with you. If you pack well, and get the right gear, you'll save time, space, weight, and even money.
If I were doing this trip through both summer and winter months, I'd bring:
- my running t-shirts (as many as I want). I can wash them in any sink, they pack very small, and they weigh nothing.
- wool jumper --- Wool can keep you warm even when its wet.
- Long sleeve cotton t-shirt.
- 1 x shorts.
- travel pants with zip-off legs to convert into shorts. Synthetic or partly-synthetic is better.
- regular trousers.
- swimmers
- 2 pairs of wool socks. They're naturally anti-microbial, keep your feet warm even when they're wet, don't stink when worn 2-3 times, etc.
- Jacket with Gore-tex of some sort, either a 2-ply Gore-Tex jacket or a Gore Paclite jacket.
- Some form of fancy fleece jacket that packs small. A softshell windproof jacket is even better. Normal fleece jumpers are the worst things to pack, but they're great because they're very breathable. They couple well to the running shirt you have underneath, and the Gore Tex shell you have on top of it. It's a perfect system.

A normal fleece jumper may take up >10% of your backpack.