Your son is going through adolescence which is often a period of great stress. Identity crisis and peer acceptance are very important issues at this age. Generally kids at this age feel inadequate and shy. As a parent you can help by-
a)Not making him conscious of his lack of friends or lack of social skills. Do not label your child as it will reduce his self-confidence. Do not compare him or criticize him and do not force him. If you are relaxed, he will relax and be natural too.
b) Encourage his self-esteem by praising his positive points. Every person does have some strengths in personality. Identify and encourage them.
c)Help him join a club or a sports group where he can learn social skills or a sport and get opportunity to meet people and make friends.