Have you tried getting in to the bath first and letting him get in with you. If you are not happy about being naked in front of him put a swimming cossy on, as he may feel more secure if he is in the water with one of his parents.
Only other suggestion is if his phobia is more to do with him having his hair washed and water and soap running in to his eyes. You can buy a 'shield' especially for children which is worn like a halo and would prevent any water and soap getting in to his face and eyes.
We have tried, shampoo shield, us in bath 1st.. in fact he will stand at the side & play with his toys with us in the bath but wont join "in the bath"
To be fair we havent tried with swimming cossys..lol
Its not the water or shampoo he seems scared of, he will shampoo mummy and daddy, its the actual "bath" or "shower" bit. it really traumatises him.
But thx for your suggestions.
I seem to have the same problem, I've done all the same things you have and haven't had any luck either. My daughter is 5 and I have been having the same problems since she was about 2. She loved her baths up until she hit 2 or so and now it's a fight everytime. Help us please anyone.