Consider 7.5 to 8 grains per lb of draw weight when 55# or less, after that you can get more in the 6-7 grain per lb of draw weight. Ones draw length can also come into play here. For your son as I did mine at 13 I would recommend a 400 spine arrow over 500 spine. The reason is that you can tune an over spined arrow but not an under spined arrow. I would have him shooting a total arrow weight of aprox 400 grains, this is ball-park! A bit more or less will not make a noticeable difference. With him in this realm he will be utilizing the bows maximum potential. I also like your choice of broadhead mentioned. Multiple bladed heads produce bigger wound channels thus maximizing blood loss for tracking! Make sure you do not hunt with either of those broadheads out of the package, razor those bad boys up to maximize cutting everything the blades come in contact with.
My son used this similar setup with a 40# compound bow with great results with a couple of bulls on the ground. Here's one of his bulls with the 40# setup, it was a 20 yard shot & it sunk the arrow to the fletchings! Bull was down inside 100 yards. Good luck, there's nothing like a father & son team, enjoy every minute with him!
ElkNut1