How do I get my kids to use the bathroom when they are supposed to?

I am planning a field trip for my 10th graders. I have one student in particular who refuses to use the bathroom when he is supposed to. He has to take medicine, and he says when he gets on the bus, he has to go to the bathroom, and he can't hold it. It's a long field trip, and we won't be stopping. I can't take him in a car just by himself. Do you have any suggestions?