T.W.
I know how you feel! My step daughter will be 10 in April and she refuses to take a bath. Like your son, she will stand in the shower with the water running for 30 minutes and hardly get wet. We have gotten to the point that we sit with her every night and make sure she washes. We bought a clear shower curtain so that we are sure that she gets clean. We also tell her the importance of bathing, but she doesn't seem to care. So, when it's my turn to sit in with her I instruct her on what to wash...wash your face, wash your neck, wash your arms, wash your body, and so on Several time when she just refuses, I will take the washcloth and wash her from head to toe, except her privates and butt, that I make her wash herself. It is embarrassing, but hopefully she will get tired of having someone sit in with her to make sure she bathes. I tell her all the time that it's sad that an almost 10 year old can't take a bath by herself, that she is acting like a big baby!!
As much as you may not like it, I would try by sitting in the bathroom. Maybe he will start taking a bath just to keep you out of the bathroom. I also tell my step daughter how she will begin to smell without bathing, how kids will make fun of her. I am even told her about how she could get infections.
Good luck, T.