DD (2.8) sometimes says she doesn't want a bath. On those nights I will give her a wash with flannel and soapy water. Would he accept that? How verbal is he? Would he understand "ok I promise you that you're not having a bath, but let's take off our dirty clothes and mummy will give you a quick clean stood there"?
Stickers for doing that then if that's well received move on to "ok how about you stand in the bath (with no water in) and mummy wash you"