Our LG went through this phase with the shower. We tried getting her to interact with it without the pressure of going in so turned it on/held her in our arms and just put hands/feet in etc. to splash. Could you do similar and see if he'll play with things in the bath but stood outside the bath and just leaning in? In the interim we used a flannel and washed her using the sink while she stood on the toilet next to it. I agree it is a phase though so it will pass, definitely keep persevering but wherever possible respect their choice to refuse so they build trust up.
My LO is suddenly scared of dark and others stuffs like vacuum noise, shadows, truck noise, all of a sudden. she was all Okay with them but now She doesn’t want to go to bed cos her fears ..
My little boy went through this a few months ago we got in with him a few times and now he’s fine again don’t worry it’s just a stage and it will pass. Maybe try getting in with him xx