I agree with Liz, I think the first place to start would be to contact his teacher and school, see if it is potentially the fact that he is being bullied or something else going on there. I hate to think thats what it is but these days, you dont know. The stress of a school day is so different now than it was when we were in school ( I strongly disagree with kindergarteners having homework, etc)
Kids cwill often change the subject because they dont want their parents to "know" that these things are happening. But if it is bullying, I would suggest taking care of it now before it got any worse.
Its so hard to see kids cry at any age - I have an 18 y.o. sister that anytime I see her upset - makes me cry!
But atleast you are in the right direction - and acknowleding his sensitivity. If the teachers dont have any sound advice for you, then I too would bring it up to his pedi? Google it online, go to the library - maybe there is a helpful book that targets these issues.
Good luck!