Just an after thought.
You didnt mention how much (if any) your son can pull back now. But if a person was to start him now shooting for next year I think you could build him to 35-40ish range. Even if he was only able to 20 or 15lbs now. Just get him to start somewhere in that 15-25 arrows 2-3 times a week, close range. Sit down, figure out a schedule, every so many weeks you up it 2-3lbs at a time. Try to have him at whatever the end goal weight is by beginning of Sept. That gives him a month before season to shoot the final weight and get comfortable at that weight. Important thing with that idea is not to go up in weight if he has hit a peak and cant pull more with out affecting his form. Im guessing he will make the first few jumps rather quickly and after that it may take some time to up his lbs.
The only reason I thought of this is becuase this is what I did for myself. I started with a 60-70lb bow, could barely pull 60lbs, looked like I was going to blow a vein in my forehead every time. I bet it wasnt 3weeks, and I was pulling the 60lbs with out a strain and with smooth movements. Then I just started cranking it up, right before season I backed my bow down a screw turn on each limb and went hunting.