Your son won't die if you don't wash his hair. When i was little, we lived in a freezing apartment that was heated by a coal stove in the kitchen. In the winter, if we didn't keep the faucet turned on and water running in the sink, the pipes would freeze. The sink was edged in a ring of ice. That was more than fifty years ago and I'm still here!

It seems to me that you are going to have to re-build trust between you and your son about hair washing. I'd play some soothing music and invite him to "watch you" and your husband wash your hair for a few weeks - ALWAYS with the same music playing softly in the background. It might be something that he likes and chooses. Ask him for suggestions. When he feels secure that you will not insist that he get his hair washed, invite him to help wash yours and/or your husband's hair. Always have the same music playing. Then see if he would be willing to wash his own hair - with the same music playing. I think it might work. You certainly don't want to traumatize him. It might even be important to wash hair in a different setting - different bathroom or even the kitchen so as to change the environment.

You might even have him help you "write and illustrate" a book for children about a little boy who doesn't like to have his hair washed. If he agrees, you might get some insight as to his fears about hair washing.