There must be something about a child's developement at that age. Out of the blue, they go from loving to get a bath to ..out of nowhere, being terrified of getting a bath. Nothing happened to cause this, just boom out of the blue..this fear of baths! What I have found to work is to tell my children when they go through this, that it is time to go swimming & play in the tub. I get special bath toys & let my child help pick them out at the store. I let my child play to get over the fear, do not push or rush or force..it only makes it worse & longer/harder to get over. I leave washing hair for last, that can always be done in the sink..if need be. (For some of my children, this is where the fear came from. Getting water, soap, or shampoo in their eyes. It was just a remembered fear that came out of nowhere. Another fear was that they would go down the drain with the water.)As soon as my child showed any indication of discomfort, fit throwing, or fear...I got them out of the tub. Try to do so while your child is still enjoying the experience. If you do not get everything washed, you can still wash your son by washcloth from the sink (not in the sink). Also when he starts to enjoy 'swimming' & playing in the tub, when he starts to throw a fit about getting washed...let him know that he will not be allowed to play in the tub after he gets washed & remove him immediately. If he lets you wash him, let him play for a little bit. The hardest part for me was washing hair when mine went through this, so the reward at the end to them is well worth it.
Also, your husband may want to start helping with 'bath' duty & introduce your son to the shower. You can work on getting him get over his fear of baths & your husband can introduce him to the shower. If it is a fear of going down the drain, then your husband can reassure your son that he is there to keep him from going down the drain (if he does not understand the concept he is too big to fit down it). Lots of reassurance & positive feedback will help your son through this. Hope this helps..