Some great advice above.
Archery in the under 10's can be a bit challenging, but they often seem to have a natural talent that puts the rest of us to shame..
Try out a few if you can. At that age I would probably lean more towards a fibreglass youth bow.
10lbs is the one to try him with. See how he gets on pulling to full draw, holding for a couple of seconds and then gently letting the bow back down. Be as clear as you need to be so that he does not let the string go - a bow going bang the first time might put him off (or worse). Make sure he does not hyperextend his elbow.
If you think that 10 is easy, try a 15.
The things to remember whith bows like these is that they are around £10. As he gets used to the bow and the muscles respond, you will be able to sell on the bow eaily and get the next one up.
Then find somewhere safe to practice under supervision and only hand him the arrow he is about to shoot - then you stay behind him. Kids like to practice drawing whilst you are up getting arrows back - they seem to be born with the need
Let us know how you get on