L.A.
They all go through this.. It can be lots of different things they are more aware of..
The water in their faces, the slippery feeling, the water going down the drain.. sometimes, they are afraid they will go down the drain.. The hot, the cold. Not in the mood for a bath when they are tired or were having fun and now are interrupted. Knowing this leads to bed time..
Does he have a bath seat? Maybe him being able to sit in the seat not facing the drain would help.
Otherwise do the sponge baths and whip him in and out as fast as possible.. Use ear plugs to take the edge off.