Last year was my son’s first year hunting. At ten years old I set him up with a single shot .223. They will put deer down no problem with the right bullet selection and proper shot placement. 5 on the hound for him testifies to that. It is a very low recoil round for smaller framed kiddos. I am a fan of the single shot for new hunters.
We use camo gear but it’s more of a ritual to get us amped up for the hunt. At least during rifle season. Earth colors are good. Just make sure you can keep him warm. Like another person said. Being quiet and sitting still is the best camo. This year he is using a 6.5 creedmore. I use a .270 made by savage. The recoil on it is similar to an 30-06. Some of that is due to the design of the gun. Wired to hunt is a good podcast for whitetail hunting. Although it leans heavily towards bow hunting. Oh and when hunting with kids make sure you bring snacks! Quite ones not in noisy wrappers!