There are some nice ideas here. Also try playing through the scenario with whatever toys and figures that you have. Make sure the bathing toy has a fabulous time playing in the tub. Start with a toy tub, if you have one, as that is less threatening. Then after a while you can move to play using the real tub, but still don't pressure him to get in. Begin with dry play, then after a while, use toys that can get wet and have them take a bath in water. Don't expect him to want to get in right away, it may take several sessions of play, first. He'll let you know when he's ready, then you can oblige letting him get into the tub. It may help if you have the characters be his voice for the worries that he might not be able to articulate yet. For example, you could have a bathing toy say, "are you going to stay right here?" Then you can respond to the toy, "Absolutely! I'm going to stay right here until you are all done!"
Sometimes a product like bath bubbles will help to distract and make the bath interesting enough that a child will forget to be upset about it. That may work if his skin doesn't react to products like that.