My kids begged me for years for a phone (whose don't) but as a SAHM I never felt they needed them. They would list of their friends who had phones and those kids all had divorced parents or were heavily involved in sports and carpools where there was a chance the child could miss getting picked up. So I do think in the OP's story, there is, if not a need, then a precedent for a seven year old to have a phone. Especially if the parents have such a relationship to only talk through lawyers. My daughter got her first phone just before she turned 12 and my son got my husband's hand me down phone this summer at age 10. And only because he really wanted a watch and needed the phone for it. My daughter, almost 13, got a watch yesterday. My son has a couple of specific use cases where a watch for him makes logical, life-skills sense for him, rather than just being a novelty.
That said, I don't agree with a locking band. What I would recommend if you really want to pursue this route is to get a Series 3 - they are now marked down to $279 in advance of next week's release. A 7YO doesn't need the larger screen the S4 offers, and not the new heart rate stuff either. Go with the S3 and save some money, and put that to a new band/case from Amazon. My kids and I each have an Umtele band/case that cost $8 on Amazon. My kids can only wear the watch to school if they wear the case, because they will fall down at recess, get pushed into a locker accidentally, have a ball thrown at them at gym. I have found that so many kids have phones nowadays that when the kid with the new phone comes in, they all fawn over it, and then get back to business. There is no stealing of phones or bullying for watches. (Although my son is going to be 11 in a couple of weeks and is over 5' and weighs 110 pounds; not too many kids try to beat him up). But phones, and now watches, are becoming fairly standard issue even for younger kids that theft wouldn't really be an issue for me, unless you live in a rough neighborhood. And the watch doesn't work without the phone, so stealing it doesn't get anyone anything.
Of course, you do have to trust your child to not trade it. And perhaps you'd want to consider the cellular version for him so that he doesn't have to carry the watch AND the phone. Yes, it would cost more per month, but it's only one thing to track, but also much less distractions with no apps, etc., but he can still call/text in an emergency. I would treat the watch as an emergency device, and not a primary device, even if he wears it all the time.