at this point, being 9 - he's got a lot of growing to do over the next few years. dont spend for carbon. focus on affordable/replaceable at a good cost. besides the fact that he'll outgrow his bow soon and need to upgrade, he's a kid, so things are going to happen and you dont want him wrecking a bunch of nice arrows.
go for basic and serviceable, which should equate to affordable and worry about getting him some nicer stuff when he gets an intermediate bow that will last him through (most of) his teen years.
i'm betting based on a typical youth bow, some simple wood shaft arrows would more than foot the need at this point.