I thought all kids did this - guess not. Mine both Did it. Nate's was fleeting. Darcy's was horrendous until she was about 18months. So two totally different kids.
When he's in the bath is there a support or is he just floating and being held by you? Nate was much happier when we reintroduced his littel cloth sling. Guess he felt more supported. We also have a bubble solution and blower in the bathroom. Mainly for Darcy but really has worked for those days Nate isn't so keen and gives him something else to focus on.
Have you tried a really deep (relaxation) bath or a really shallow bath that he can just lie flat on his back in? We used to just fold up an old towel into quarters for Darcy and lie her on that in the bath. That had some success for a bit. Only had 5-10cm water. Some kids are extra sensitive too. Darcy's idea of too hot is way colder than my wrist temp says is ok.
I gree with the others though. Just give the baths a miss for a little bit and give him a wipe. It's not like he's really dirty.