clean your bathtub really well, take out any anti-slip gear, fill it up with about 3-4 inches of warm bath water, let your son scoot around on his back and sit with him. I used to sit on the side soaking my feet and singing to my daughter (I think rubber ducky was the song) and she would just shoot up and down the tub zipping around on her back like a frog, I actually filled it deeper, but I started extremely shallow to see how she would react. It was VERY positive. I began to do this really early, she was about 1 month. VERY cute, you will die it's so adorable. I would sponge her body and hair off with a baby magicked washcloth when she slowed down for a few seconds, and then she'd kick off and scoot to another part of the tub. Hilarious.
Well, as long as you son really digs water, he would really like scooting around on his back in a few inches of warm water. Obviously I don't need to say that you never leave the tub in case the child flips over face down (mine never did, but you never know.) Have fun!
PS we took showers with her too. The trick to that is wrapping the bottom in a wet washcloth so it's not too slippery. :o)