What kind of trouble has he been getting into? Is he talking in class and being disruptive generally? Is he being disrespectful to the teacher? Picking at or arguing with other kids? Shooting spitballs? What is he doing that is getting him in trouble?
If he is finishing his work early and getting into trouble because he is playing around (out of boredom?) then perhaps he should be evaluated for giftedness. If he is not finishing his work and getting into trouble for daydreaming out the window or for constant distractions (up 15 times to sharpen his pencil and playing with erasers from his desk) then perhaps he might have an attention issue. Sometimes this can be a simple matter of him getting more sleep or a higher protein value in his breakfast and lunch to smooth out his blood sugar fluctuations throughout the day. It could be more than that.
You didn't really say in what manner he is in trouble, so it is hard to give very good quality advice. Sorry.
If he is crying... could there be a bully? That could also cause him to act out in ways that might get him into trouble.