Like many others have said, if this is a true, true issue and he is not at all capable of "holding it", you have to get your pediatrician involved. Not just from the school perspective, but because this could have longer (and larger) medical implications as well.
You mentioned other types of behavior that you and the school have been working on to help your son...has he been given an IEP? I ask because if so, then the bathroom issue (supported by a doctor) can become part of his IEP, which is a legally binding document, and then both you AND the teacher would have a leg to stand on as far as making sure he gets to the bathroom when needed.
Is he young for his class? Some of the concerns could also be maturity related. Just an idea.
Moms want what is best for their kids, and teachers do as well. I also don't buy that teachers who are moms are more understanding than those who aren't....my son's first grade teacher was a mother and she was not a good experience for him. His second grade teacher is not, and she has been nothing but an awesome experience for him. My son got in trouble daily in first grade with a lot of the things you described for your son (curious, keeping still, talking out of turn, etc...) and we've had one incident only so far this year in second grade.
Teachers don't set out to oppose parents, but kids are so different at school and in public. Remember that teachers and coaches see things we as parents don't (or won't) see, and their observations are just as valid as ours as parents are. We know that throughout life we will need to wait, or conform, or adjust what we want because of the situation we are in. Kids need to learn that lesson too, and school is where they learn it. If teachers let kids go to the bathroom, or the office, or in their book bag, or whatever, whenever they wanted, no teaching would happen, and we as parents would be crying foul that our kids weren't learning. Yes, it's a trade off, but it's a very necessary one.
Talk to your pediatrician, see what he or she can do for your son in the way of a medical diagnosis, then go from there with the school.
Good luck!