Not an easy order because in general, the ones that try to be like Halo kind of fail to get the story right and leave out a lot of things that make the Halo games. It's not going to be as good as Halo, but the closest games to it is Nova 1 and 2.
The other FPS games are generally different licenses altogether.
Nova 3 blows Nova 1 and 2 out of the water, but the thing is, the series goes away from the Halo-style gameplay and definitely jumps to something more akin to Crysis 2, in the third iteration. That might be a deal breaker if you're specifically looking for something that's a partial Halo-style experience. Just remember that none of the games actually have all the elements that make up Halo in gameplay or in story.
However, I only mention Nova 3, because the presentation and the story make a major leap for the better, IMO. It is that game that finally has the series really come into its own. I would say it could be iPad's Halo only due to the significance of it appearing on the platform. By coming up with a combination of ideas, and unique presentation, that brings major attention to good FPS gaming on a new platform.
Just don't have him go into Nova 3, if he's the one that says "Halo or bust".
Nova 1 and 2 come closer to mimicking certain aspects of that license, but at a more generic and less-featured level. Though Nova does have a few neat aspects like the opening of doors, grenade play, and multi-creature targeting made specifically for the touch screen. Those 3 were probably the most inventive ideas brought to the Halo-likes, and were the main gameplay aspects that made Nova unique for its time.
Short version: Nova 1 and 2 have the most Halo influence in their look, weapons, and environment. A few inventive "made for touch-screen" controls are the unique aspects added onto the games that otherwise may seem like a Halo-light. They aren't bad, but a bit of a disappointment if you're expecting Halo console quality. Nova 3 nearly completely diverges from the Halo style but greatly ups the game quality. So if someone was willing to be more open with the atmosphere and environment of the game, Nova 3 is there to blow you away and also make you think someone crammed a current gen console into a flat device.
PS. There are other First Person and 3rd Person games that are pretty nice on the system, but they go for completely different licenses and gameplay hooks. Some being actual games from the same publishers of the big console licenses. Completely different gameplay from the ones I suggested above, but you do have the likes of:
Call of Duty: Zombies (Decent version of the Zombies stuff of the console version)
Mass Effect Infiltrator (Pretty good, maybe not as well executed for input-controls as Dead Space, but is a well-playing looker (once you get the hang of it) that takes good advantage of the New iPad. It is its own story and game eschewing almost all the RPG elements that made the console Mass Effects, and changes it to a cover-based shooter complete with Biotics.)
Dead Space (Horror-themed Space shooter, but an extremely well made game that is both console-quality and contains well-executed touch-screen controls and interface. Like Mass Effect Infiltrator, it is also its own game with a unique story-line that is canon in the universe the other games are in).
No problem. I've followed Halo through the years, since the first came out. So I'm quite familiar with its elements. I figure I would lend some suggestions about the various FPS and certain TPS games on the system, which might hold similar interest. iPad has a number of them, and they are growing, mostly because of iPad's power being high enough to run good engines like Unreal, etc. And that they seem to be getting a decent bit of commercial success.