Is it okay to expect a 10 year old boy to use a diaper when pulling over the car is inconvenient?

My son has full control over his bowels and bladder, but he has epilepsy, which is mild and frequent despite medication. About once every 2 or 3 days he will have a small seizure, and while it lasts only about 15 seconds, there is usually no forewarning and he loses bowel and bladder control. So he wears a diaper for this (a type of pullup). The other day we were driving on the highway, and already running late. I had to drop him off at home and then get to work and I was already late and he told me he was desperate to use the bathroom to pee and I needed to pull over. I asked why he didn't go before leaving and he said he didn't have to go then. Plus he drank a lot of soda pop. I told him we couldn't stop and that since he had on a diaper, he should use it. He started to pout, but I told him it wouldn't change my mind. He ended up not being able to hold it and he peed in the diaper. Now he's mad at me. Am I being unreasonable to expect him to use the diaper? I figure that since he's got it on, he might as well take advantage of it.