The ball, not so much.  Rule of thumb is one pound for each year old.  8-Pound for an 8 year old.  If he is bigger and can handle a heavier ball then that is ok.  Plastic/Polyester is best for a beginning child.  Just teach him safety and courtesy first.  I mean, don't walk up on someone when they are next to him on the approach.  Step off the approach after he thows his first ball.  Don't step over the foul line because of the oil.  Don't pick up the ball in a way that he might get his fingers smashed between the balls.  Slowly work him up from the one step release, to the 4 step approach.  Get him to knock down pins.  Good luck.  BTW, if he won't listen to you, then he might listen to a coach...just sayin...

Thanks for the advice.

Great advice from everyone. As far as ball choice goes I would look into a Storm Polar Ice. They are basically the same as plastic/ployester performance wise. The difference is the Polar Ice is urethane and have shown to be much much more durable. They are a few bucks more but when you factor in the cracking issues with light weight plastic I think the few dollars is well worth it. Either way you cant really go wrong. Just giving you another option.

Thanks again.  I had asked in the AMF section about the possibility of plugging and re-drilling either of a couple of old AMF balls I had, but given how old they are, posters said probably not worth it.