How about letting him lather it in but start out with him washing his hands arms and slowly work on up when he gets to the point where you ask him to wash his hair hold a mirror up so that he can watch himself wash his hair. At this age anyway is when children start learning how to bathe and washthemselves anyway. So why not include his doing his own hair just like his washing his own face and other body parts.

I had a friend whos child was afraid of his parents washing his hair because it wasn't something he did on his own and when parents don't allow children to do something on their own its because whatever it is they cant do usually has an element of danger or unsafeness in it so this child naturally felt it was dangerous and unsafe to wash his hair. When I took that element out by having him do it himself with a unbreakable childs mirror he was no longer afraid of hair washing.