First of all, make an appointment with your son's teacher. Good communication between home and school will be key to his progress. Describe your son to the teacher as you see him at home, and let her know how your son behaves when he becomes frustrated and doesn't understand something. Let the teacher know about any approaches that might be effective in getting your son to ask for help. The teacher might in turn be able to give you insight into your son from a different perspective, and she may have her own ideas about how to help him. The important thing is to open the lines of communication. You and the teacher are a team. The more information you give the teacher, the better equipped she will be to teach your child and to encourage him to be more assertive.
Secondly, encourage your child to become involved in a leisure-time activity of his choosing. Find out what your son's real interests are, and sign him up for lessons, a team, or a club. It isn't necessary for your son to do what all of the other kids are doing just because it's popular. But it is particularly important for shy children to pursue extra-curricular interests because it gives them confidence in their own abilities and practice in social situations. Your son might be reluctant to participate in an activity because he is not sure what to do or feels inadequate. By participating in an activity he likes, he will gradually learn that he has talents and can feel more comfortable with others.