my ds went through this.
I sat in the bath with him and a new toy he loved, and played with just a cup of water, didn't get it on him other than toes when it was being played with.
Second time i bought a flannel in with us, did the same thing, bought a jug and a cup and played again, only we got a bit wetter, and we used the flannel to 'mop up' and i pretended he had a smudge on his hand so used the flannel to wipe it off.
Over the next week we did this a little bit every day, and once he would get in to play, i started using the taps to refill the jug, popped the plug in an let some water gather in the bath with us.
It really was just patience, reassurance, and tiny step/making it fun.
tbh, he never really took to having baths and we ended up switching him to me getting in and doing a mix of a couple of inches of water and the shower head.
I will say some of our problem is he is autistic and even at 18 still can't stand water on his face or clothes, and will not take a bath. he showers and i still have to help wash his hair as he has to hold a flannel over his eyes to keep the water off.