My middle son went through this phase - but his issue was all water - he refused the swimming pool and shower too. He's 4 now and well grown out of that phase but at the time it was an impossible phobia that drove him into hysterical fits and hyperventilation at times.
All the responses below should work well if your son is just going through a 'little bath-phobia' so try them first.
If it is extreme, here's what worked for me:
1) Buy some waterless soap and shampoo for those days he just can't take the bath and you don't have an hour to spend working him into it. These are meant for people who hike/camp so they are easy to use (rub on, dry washcloth off) and as an added benefit are hypoallergenic and safe for the environment. I would clean him in my bedroom so he felt safe and relaxed. Note: I only used this out maybe once a week. Sunday mornings were usually really hectic.
2) Stage waterless playtime in the bathtub. Bring in washable markers, watercolors or other fun stuff he usually can't play with and let him go at it. You can just rinse it out later so no worries on clean up. Keep his usual bath toys in there. I would even encourage my son to color on the toys (which were cars, btw) so that after playtime I could tell him they were all 'dirty' and playtime was fun but they needed to be clean. I'd have him wash them with soap, water, and shampoo in the sink and monitor him closely, talking to the toys like they were people and telling them how nice and clean they looked and I bet the shampoo and soap felt so nice. I didn't talk to him directly or leave room for him to argue.
Later that night I'd stage the playtime all over again but when it came time to end I'd say how dirty everyone was and run just a little water in the bottom of the tub so he could clean the cars and himself. I'd tell him the cars wanted him to be clean too and he could tell him when there was enough water.
So you start out with only a couple inches of water and each time push the envelope until he can handle a full bathtub. It took me about a month before there were consistently no issues with bathtime but by the first week it was tons easier already.