I have had a Malibu Mini-X for a long time, 9' long, about 45lbs and at 10 he should get years of use out of it. This is very stable, it was hard to flip even in a pool. I have used it myself and I'm 6'4 and a little heavier than flint (I blame the scale). Both of my kids have grown up using it as a first kayak. My son is 11 and my daughter is 14. The only draw back this yak has is it's wide so it is a little more paddling than a narrower yak. They both used it in ponds and the Canoochee river with no issues, but we took it out onto the flats off Tarpon Springs and in the current it kicked my sons butt. Everyone has their own preference. I prefer a sit on top for the kids mainly because it is an easy in and out for bathroom breaks or a swim call and if it does roll we're not stuck in the middle of a lake with a hull full of water. Whatever yak you end of choosing I hope you both enjoy it and the time on the water together.
NUCANOE Pursuit
Kaku Voodoo
Wilderness Ride 135
Old Town Canoe