There are many reasons why some kids don't like to get wet or dirty. Some children are simply neat they like their clothes clean and their toys in order. Others dislike being wet because they're learning to use the potty (or have recently learned how), so they worry that being wet means they're "babies." Others don't like the feel of wetness, and they prefer being warm and cozy. And so on.
You and your child's healthcare provider know your son best, so you'll be the best judges of what's behind his behavior. Does he also complain about being wet during bath time, or is it just about his clothes? Does he enjoy things like finger painting, or does he dislike the mess? Talk to your provider, and see what he or she recommends.
Once you pin down the issue, you'll be able to help your child discover how much fun water can be. Activities like playing with toy boats during bath time and experimenting to discover what floats or sinks in a bowl of water are great ways for him to get used to water and they can be a lot of fun for both of you. In the meantime, when he does get wet and gets upset, try encouraging him to put dry clothes on by himself (with help from you as needed).
