Ok, this might be an extremely stupid question, but here it goes anyway!
DS is 15 weeks and loves the bath. Screams when you take him out and falls asleep in it 
However he bloody hates having his hair washed and this is quite a recent thing. I've always done it the same way..... Lie him in his hooded towel, swaddle him up like a sausage, hold under one arm with hand supporting neck and arm supporting body, other hand actually wash his hair. Then when done whip the hood up, dry hair then put him in the bath.
Now he's bigger should I be doing it another way?! Saying that though, he's still a tiny little 9th percentile thing, only 12lbs so not very robust!
yes, I am a FTM just in case that wasn't screamingly obvious!
babies should come with manuals, like cars and new washing machines do