Is it 20# at his draw length now? That is already pretty heavy...I would say too heavy for most 5 year olds. Heat treating will increase the weight a bit. Being overbowed can lead to bad habits, and will not be as fun for him. If you know he is comfortable with 20-25# draw, that is your call of course...if so, you can start making him a warbow for his 10th birthday!
If it were me, I would tiller that one down to 12-15 pounds (or less) at his draw, take the other half of the branch if you have it, and make myself a heat treated short bow as an experiment.