Has anyone else had this problem?

I am the mother of a 4 year old with Down Syndrome. Ever since he was an infant, he's had problems with cradle cap. I would comb out the flakes, then make sure I rinsed really well when I washed his hair. No matter what I did, the cradle cap would come back. Two years later, my son still had it. My husband seemed to think it was because of his hair not being rinsed out good enough, even though I explained that he's had this since infancy and rinsing well didn't stop it. He put my son under the shower to rinse his hair and he screamed in terror. Ever since that incident, my son started crying hysterically whenever I tried to give him a bath or wash his hair. Though he's always seemed to dislike the sensation of water running over his head and down his back, it was to where it was nearly impossible to bathe him correctly. A few months ago I tried covering the shower head with a towel and it seemed to work! He got in the tub, played with his toys and though protested at having his hair washed, he at least wasn't screaming anymore. That changed last week when I gave him a bath. I covered the shower head as usual and he climbed in with his toys. But when I went to start washing him, he started crying hard and wanted out of the tub. I've run out of ideas and it would be extremely helpful if someone out there has had this problem too, especially with a special needs child.