Perhaps he doesn't like tub baths. If so, he's not alone- a lot of people I know don't like them, including me. I prefer to shower, and perhaps he does too- and there is NOTHING WRONG WITH THAT. Has your son been taught to DO all that he supposedly is expected to do? Has his dad ever been in the bathroom with him to make sure he's doing what he needs to do to maintain his health? Your son sounds as though he has a case of psoriasis, which is a hereditary condition that causes chronic, itchy, flaky dry skin, especially on places like the elbows and knees. Psoriasis can cause sores as well, especially when the skin in the affected areas dries out to the point where it may crack, bleed, and become infected. Frequent bathing only exacerbates the condition, which is why doctors who treat this problem may recommend that the patient take sponge or sitz baths during flare ups. Have you talked to your son's doctor about any of this? Does the doctor know what's been happening?
Perhaps your son is allergic to the soap or shampoo he's being asked to use, and that's one reason he avoids bathing. It may hurt him. If this is the case, you need to ASK him if there is an issue with it, and you may need to change brands of shampoo or soap until you find one which he can use comfortably. It may be that your pediatrician will recommend a medicated soap and shampoo that won't irritate his already sensitive skin.
I would look into these things I mentioned before you start disciplining your son any further- he may have a legitimate reason for avoiding baths. Ask him if he prefers to shower instead, and LISTEN to what he says to you. Once you find out why he's really avoiding bathing, then it will be easier to address the situation. If it's a medical issue, then he needs to see the doctor, not get yelled at by you. In the case of behavior issues, you might want to consider letting HIM decide when he wants to bathe- many people prefer to bathe in the morning before work or school, and again, there is nothing wrong with this kind of a preference. There's no law out there that says that everyone must bathe at night, regardless of whether they want to or not, or even if it's practical or not. Just make it clear to your son that he is expected to bathe and shampoo his hair at least once EVERY DAY, and leave the time he does it up to him. Let him know as well that if he has problems with something ( like reaching above his head to shampoo his hair) that he can come to you or his dad for help. By making him take the responsibility for his own appearance, you will be eliminating the cause of a lot of your conflict. It's okay if he doesn't want you or his dad in the bathroom with him- at his age, he's beginning to develop the sense of modesty he will have as an adult, and that's normal. You can point out to your son as well that if he doesn't keep up his appearance, or he has a bad odor that people can smell, it will cost him friends and make him an outcast, because no one wants to be near someone who looks or smells bad. Most boys his age will do anything to keep their friends- at times, those relationships will matter as much as or more than his relationship with you or his dad. If you point out that not bathing is going to cost him friends, then he may rethink what he is doing. Good luck, I hope this helps.
I am a medical coder who works in the health field.