I would think in any state it would be leagal for a 7 year old to hunt what ever he wants.
As long as he passed a hunter saftey course of some sort & is accompanied by an adult.
I'm all for teaching kids to hunt.
Now with that being said, I'm not a fan of using a 410 for deer hunting. I know a lot of deer have been taking with them but its just not a good choice. I don't know for sure but I think the slug weighs one fifth of an once? (87,88 grains)
Get him a 20ga. I believe they make low recoil slugs for it.
H&R might make a 20ga. Slug gun you could get cheap.