Washing baby’s hair

I am finding it difficult to wash my baby’s hair while holding him like a football hold over the bathtub when we give him the bath. I support his head with the right hand and wash the hair with soap and brush with left hand. But these days it’s becoming difficult as he is moving his head a lot and as my right hand has soap it is slippery at the back of his head adding to his movements.. also he is gaining weight day by day which makes it even more challenging to hold him for good 5 minutes with all this going.. anyone have any suggestions or how are you giving your baby a headbath.. in what position?