I would avoid Planet Hollywood. I've heard nothing but terrible reviews. My son ate there on a school trip once and said it was the worst meal he'd ever had at Disney.
Since you mentioned your son and not getting into T-Rex, have you thought about Rainforest Cafe? I know, I know (for everyone rolling their eyes). It's over priced and the food is just so-so. But it's not horrible, and when our son was that age he absolutely loved that place! We ate at quite a few different ones when we were traveling. We never left hungry and he had a blast. An added bonus is there are two of them at Disney, so it's probably a bit easier to get into than T-Rex.
Landry's (parent company of both T-Rex and Rainforest Cafe - as well as Yak and Yeti) has a membership program that supposedly gets you priority seating for walk ups. We aren't members, so I can't speak to how successful you'd be. I've seen it mentioned a lot on these boards though, so if you do a search you might find some threads talking about it.