This is just the way I look at it.
A boy needs a .22 and a shotgun. They just do.
When it comes to "what kind?", I say, get one that won't be useless to him when he's older, but won't be so complex, or valuable that him, being a kid, won't ruin or devalue a complex or high dollar firearm. Also, a kid needs to kind of pay his dues. He shouldn't have the very best things handed to him. He should start out basic, and have to earn the finest things, if he wants them.
I think a quality, preferably older lever action, or bolt action .22 is perfect. One that shoots shorts and long rifle. I think a sxs, or over and under 20, 16, or 12 gauge is best (I was shooting 12 gauge at the age of eleven. Boys are tough. Don't treat them like babies.
).
With these choices, when your son is 30, they won't JUST be keepsakes from his youth. They could also be useful guns, that not only please him to have in a cabinet, because they were from his childhood, but please him still in the field too.
I know single shots are useful. I just never use them, as an adult. My dad gave me a 12 gauge, and a bolt action .22. He said that's all a young man needs (I was just 11, but already proved I could handle the firearms he gave me.) I still have them, still use them, and am so proud to show them off. They aren't worth much money, but are fine quality firearms, that a boy could handle, and a man can still use.