If he's been riding your DH's bike, does that mean he's big enough for an adult bike? There are some spectacular bargains to be had on smaller mens frames if you buy them second hand - I know it maybe doesn't sound like a great Christmas option to get a second hand bike, but for $300 you're looking at the difference between a basic frame with cheap componentry, against a solidly built frame with top quality components that will last a lot longer and require less maintenance.

2 of my 3 current bikes, and 4 of 5 I have ever bought have been second hand - it lets me get a lot more for my $$$.

But if you have to go new, then I'm on the shop's side.