You can do a lot in this game, or a little. He's 5, so he'll probably stick to the basics. He'll eventually need to deal with inventory management and Pokemon storage. And, he can be excited for the occasional shiny that will come his way.
As for the gym, once he reaches level 5 and chooses a team, he can try to take it down (unlikely at that level), or drop Pokemon if it's the same team and has fewer than 6 defenders already. When it gets kicked out, he'll get some coins, when over time will let him decide what to buy out of the shop.
Raids are going to be too much for him without a lot of outside help.
At that age, he's going to do a lot of things that we adults would consider mistakes - deleting shinies, evolving low-IV things. Don't let it get too complicated for him.