In March Ni'ihau is likely still shutdown (big weather) Tunnels probably also since it's on the north shore.
Nearest shop to you is Seasport Divers in Kapaa but their main shop and boats are kept in Poipu south - about a 20? min drive. Fathom Five is also nearby.
Should a window open up to get to Ni'ihau, Bubbles Below might go out there but they're in Port Allen across the island from Lihue, I would not count on it but you might call the night before, Not for your son, it's an all day trip with a rough crossing and you share sites like Vertical Awareness deep with resident reef sharks,
Jeannette at Fathom Five is a local Tunnels expert if that becomes an option - if it does go with them , parking is limited and it's in a neighborhood.
Koloa is a decent dive. Old boat ramp south near Poipu to walk down and there can be interesting stuff there - I've seen octopus plus a lot of fish.
Sheraton Caverns is the boat dive for both of you, IIRC not much over 50' and a lot of it shallower. It's a series of collapsed lava tubes and any overhead sections are short. Bring a good light, shine it on the roof and watch for movement. Massive green sea turtles also live in the ledges there - we got right next to one for photos while it slept the whole time.
I also liked the Fast Lanes dive site but it might be too much for your son.
Seeing the humpbacks is really cool, No diving near them, they're protected. Poipu Beach is really nice area - there's a couple of beaches near each other. Where ypu're staying tends to be rockier, It really is the Garden Isle - very green and lush.