Hubby is 5'10" and when we got home from our trip he was 299.4 (he's been doing WW for a couple years so he knows exactly). He fit on Screamin'.
Now he has a broad and thick back, and his arms are big, so when he put his arm up to pull down the harness thing it got stuck up, poor guy. He let the CM know, the CM let the harnesses up, and DH tried again.
He was worried about not fitting, since he still can't do Maliboomer (they have a shoulder harness like Screamin' but then it has a tether to clip into the seat, and he's an inch, inch and a half away from the tether clipping), but since Screamin' didn't have the clip he was fine.
Your son will be fine.