Children talk to adults when they feel safe, loved, and close. You can give your child that sense of contact either by playing with him vigorously and generously, or by listening to him without judgment or interruption. Your child may also be more willing to open up if you ask him a positive question first. You'll hear about what's hard at school, but you'll have bypassed the hopeless feelings that can make children reluctant to talk. Let your child fall apart.
Children keep a lot inside but are always looking for ways to get their feelings out.
Children will get very particular about a small thing because they have a little volcano of feelings inside that has nothing to do with what they're getting upset about. But it's the only way they know to address what they feel. This won't be easy for you as a parent. You may be every bit as cranky as your child at the moment he picks to fall apart, or you may be under a lot of pressure to get something done. But your child will benefit tremendously if you can go down on one knee, put an arm around him, and listen while he cries as long as he needs to.