How old is he? Have you tried to give him some one on one time? Yes some kids are more sensitive than others, and some are more closed up than others and need more encouragement to open up about their feelings. It is however important to spend time with them and allow them to talk about their feelings and what is going on with them, both at home and in school. Let him tell you what HE needs and why he seems to feel unhappy.
Riding in the car is one of the best times a kid opens up. Also bath time when they feel relaxed. It is also important to give each child some one on one time with each parent. So if you can arrange to take him out for ice cream, for instance, and just give him an hour one on one, he might start to open up about his feelings and his needs.