In terms of string options, I say goodonya for keeping your young slugger-to-be away from the poly. I've been coaching high school teams for several years and I've also been stringing for just about that same time span (well over ten years). Some of the real killers among the high school ranks who are college players in the making can benefit from that stuff, but somebody like your son simply won't - not for a while.
Yes, even young players with really strong technique can get wrist and elbow injuries. Through my coaching & stringing years, I've seen at least a dozen of the stronger local boys and girls need to switch away from poly into softer string after one issue or another reared its ugly head.
That being said, you may also want to try some good ol' synthetic gut in your son's frames. I personally love it in my racquets and I also use for lots of the locals who have me stringing their frames. Multi is a very good option for players who need an especially soft string type and don't want to get into the expense of natural gut, but I've found that multi can steadily degrade and soften up as we use it.
Syn. gut is moderately soft - fine for the vast majority of us - and it's not as prone to the same degradation as multi. It's also easy on the wallet. If your son really gets into it and starts gobbling up strings more quickly, you can easily just buy a reel of what he likes and then pay a stringer for labor. I do this for a few folks I know - it's nice for both of us because I know they they're getting exactly the string they want.
A 98" or 100" head size is just fine for anybody I think, including a beginner. I'm not much of a fan of a "beginner's frame" that somebody will grow out of rather quickly after starting out, unless we're talking about a smaller kid who maybe needs a shorter racquet length that's appropriate for their size and wingspan. I think it's smart to get your guy settled in with something he can use for at least a year or three. If he's mishitting with that Warrior, he's messing up his move on the ball. A bigger head will only reinforce bad habits.
As far as the allure of the bling?... The only thing I'd say there is maybe let him buy his own racquets with his own money. Or after you get him set up for now, you could tell him that any future switches will be on him. That way he'll probably be extra careful and deliberate with his choices. If it's his money, he'll probably be less inclined to treat his next racquet like a fashion accessory, right?