It is too much to expect him to do this on his own - you can already see that such an approach is not working. There's no point in continuing to do something that is not effective. The timer idea might work, if it can be programmed in such a way to sound an alarm periodically. If not, it won't be effective because it would rely on him to remember, which he won't do.
I have a parent teacher conference scheduled for this Monday, so I will ask her to do something like that with him. I don't know if it should be her responsiblity to remind him though, I think he is old enough to know better. I really just think he has a weak bladder, as do I(never really diagnosed of this though) and he inherited it from me, which is why it is important for him not to hold it too long otherwise when he does laugh he will not be able to control it. As from my own experience, I just went all the time whether I felt like I had to go or not. I told him he should go at least 2x/day at school, especially before recess. The only other thing I can think of besides asking the teacher to remind him is maybe some kind of watch with a timer???
What is your own sense of why he hesitates to use the bathroom? Obviously his explanation is not credible, but what is your own judgment? At school it would probably make sense to have the teacher arrange for him to use the bathroom on a schedule several times a day for now. Finally, is there anything from your own experience that you think might help? How did you solve the problem? Were there any interventions from your parents or teachers that you found helpful?