My first first white water boat, I was choosing between a Perception Shock (42 gallons) and Siren (40 gallons) both being playboats (old models) with slicey ends. I am 5'3" and 140. I took the Shock and it was a good choice for me. I learned on class II with that. Did a lot of accidental stern squirts but didn't matter to me since I already had a roll. Learned to handroll with that boat. If your kid is 80 lbs, he has a bit of leeway and forgiveness with the Siren. I think the only thing smaller is if you go with a Jackson Fun boat made specifically for children.
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PS. The RPM is way too big for my taste. Having paddled and owned some semi planing boats, I much prefer the planing hulls like the Siren.
Reaction from someone who is a newbie at short boats - I had my Piedra, old style displacement hull for small people, in moderate moving water last fall. It did the ferries, peel-outs fine and forgivingly in uncomplicated moving stuff we were in. Whatever lousy results were the paddler's skills, not the boat. It is a fantastic boat for learning a roll, some of the off-balance stuff. The older displacement hulls like this and the Pirouette are the only WW boats I've been in that roll as easy as the Romany or our older DS Elaho. It also has hull speed of some sort, so on a straight pond paddle would not cause me to want to commit hari-kari. In sum, it more than deserves its respect.
But we picked up an Inazone 220 for me a couple of months ago, an older planing hull design that was popular as a school boat, and after several sessions with it in the pool my eyes are ope'd. I would not want to take this boat out on a straight pond paddle, wouldn't put a newbie in it unless they really wanted to work on their stroke and would use the Piedra for most people learning a roll. But I find the Inazone's edge control so much more sure even in the pool that I wouldn't think of taking anything else out into moving water. (which first shot was supposed to be last night but being at full flood stage was too much) I really feel like this boat is likely to be safer than the Piedra with its control and primary stability, as long as I can avoid catching an edge.
So - for your son - I am not sure what kind of paddling you want to start him off with. But if it seems like the first shot will be to put him on a bit of moving water where he can work less of paddling for forward motion, I'd look around for an planing hull boat for smaller people or kids used. And given his age and athleticism, he may get a roll faster than a grownup would anyway.
I just got a Jackson Fun 1.5 a few weeks ago. It's my first whitewater kayak and so far I've only had it in a pool and a lake and love it. It's really comfortable, stable, and amazingly easy to roll. I'm a bit bigger than your son ( 5ft and 110lbs) and besides the smaller Fun 1 it's the first kayak that I've ever been in that is truly designed for smaller people.