H.V.
My 9 year old went through something similar recently. She was going very often...to the point where her teacher was concerned about a UTI or something to that effect. We went to the doctor and all tests came back negative. The doctor suggested it was anxiety. My kids is a bit nervous by nature, nothing in particular was stressing her out- but she has a busy full life with school, sports, little sister, etc..
We've worked really hard on having a little more peace at home...just trying to not have it so hectic...especially at nighttime. She has DEFINITELY improved since we've mellowed out the home front.And the teacher set limits on how often she can go at school.
Maybe your son has a bit of anxiety. Or maybe he is like my brother-in-law and just has a dime sized bladder :)