I wash Mr. D in the shower - we have a detachable nozzle and it has a door, so I go in there with him and practically get a shower myself. He's too big to mess with in the tub. It can get annoying because he doesn't like it and tries to edge to the edges of it where I can't reach him as well. Because the shampoos we use have to stay on 5-10 mins, I hang out with him in there and sing to him too.
I try to do it when he's mellow and not hyper - maybe see when your baby is sleepy and try it then?