If you do go with a 410, check out the versa pack. It is youth sized and comes with interchangeable 410 and 22lr barrels and includes a very nice padded case which holds everything.
I picked one up a few years ago for my wife for around $125 (I use it more than her, though, as it is a fun little gun to shoot even though its too small technically). I'm not sure what they go for now, but I think there was a post here or on the rifle board not too long ago from someone who ordered one through Walmart for not too much more than that. Its a pain in the butt to go through Walmart cause noone there knows what they are doing, but they do have the lowest prices. Also, Gander Mountain can order them pretty cheap, and they know a lot more usually than the Walmarts.
I am not sure, but I believe you may also be able to add shotgun barrels of larger gauge to the receiver when your boy does get a little bigger, although with the prices going up on the accessory program, you can just about get a whole new pardner for the price of the freight/barrel/fitting charge.