Does your son like community’s with lots of little fish swimming around, maybe some shrimp. If so, I would suggest getting Chilly Rasboras some cherry shrimp, and a snail. If your son likes 1 big center piece, I would suggest a betta. I love their personally, and I have had at least 1 betta at all times for years. If he wants a mix of both, I would do a honey gourami. They are peaceful, and have wonderful personalities. Also they stay small, and you could do 6 pigmy Corys and 1 honey. And you have a beautiful tank with lots of action.