M.K.
Hi L.,
If you've tried everything, you may have tried this too, but... my (now) 23 month old went through a similar phase and I would soap up his head quickly and then have him lay down in the tub on his back. I used a wash cloth (a nemo one that he likes)to gently rinse out his hair- no direct dumping of water. To him it just felt like I was gently massaging his head with the wash rag. He was more afraid of the water than the washing, so this did the trick.
Good luck!