D.,
Oh lord! The things that freak them out. It's amazing how long they can hold on to these phases, too.
For hairwashing, you can sit him naked on the sink if you've got a sprayer, and just wet, soap and rinse there, without him having to get in. Be really careful, they're squirmy.
As for getting him back in the tub, try letting him play in it without any water once or twice. Get some tub paint, and let him decorate the wall if you have tile or a tub surround. My little guy loves it if we'll blow bubbles for him while he's in there. You can put some water in the tub and let him float some plastic boats from the outside, just to be comfortable playing near or in the water.
Would he stand in a shower with you or your husband? He might feel better if he's got a parent with him. Do you have more than one bathroom in your house? Sometimes even a small change like using a different bathroom will stop the refusals.
Best of luck!
Jess