Regardless whether your son is small framed or built like a tank, if he has never drawn a compound bow before he is going to want to start at a fairly low poundage like 35 to 40 lbs. The problem with this is that if he practices a lot then he will build strength fairly quickly. I would be looking for a bow like the one in this ad that grows with the shooter. Also if he decides that bow hunting is not for him then the bow has already taken the depreciation for re-sale.
http://www.outdoorsmenforum.ca/showt...hlight=diamond
https://www.cabelas.ca/product/76956...ow-rak-package
I would then take the bow to Jimbows and purchase the arrows there and pay them for a couple hours service ($100), to fit the bow to your son and give him some basic shooting lessons.
You will be well within your $700 budget.