How do you get a boy to bathe?

Today as I was doing laundry, I came upon a stack of clean underwear, boys underwear. I started sorting it. John's, John's, John's, John's, John's John's, Scott's, John's, John's and John's. This is about a weeks worth of laundry. John has so many because I change his underwear everyday and he has several accidents during the week. Scott, on the other hand, had one pair, ONE PAIR. I have a very difficult time convincing him that he needs to take a bath and I really don't push the issue unless he looks or smells dirty. He absolutely has a bath at least once a week. Some of you may think that is gross, but like I said unless he looks dirty or smells I don't have the energy to fight it. With diapers and accidents, John and Kate have baths at least every other day. We are going out of town soon and are planning on swimming. Scott told me he didn't want to go because he has to take a shower or bath after he swims. That is how much he hates bathing. I have told him several times that he needs to change his underwear at least every other day even if he doesn't take a bath. Clearly, he is not doing that. I have heard that things only get worse in the hygiene department as they get older. What am I going to do?