Your son has done really well to go Cold Turkey, and also to open up to family and friends about his addiction. It sounds as if he may be shy and have low self-esteem and self-confidence, which is why perhaps he was so attracted to the drug in the first place.
Could you perhaps persuade him to develop some new hobbies which would increase his self confidence - joining a walking group, a drama group or even doing some volunteering (your local library or Council will probably have a list of opportunities) which would help increase his sense of self-worth. Some Rotary Clubs or the Lions have volunteers groups for younger people where he could mix with others of his age group in a social way. Try Googling "Volunteering opportunities + your post code" and see if any interesting ideas crop up.
Would he consider going to a cookery course , or could you give him and his friends some cookery lessons, concentrating on one dish which they all sit and eat together afterwards as a social occasion? Any hobby where he learns new skills would help increase his confidence.
Meanwhile, keep telling him how proud you are of him for kicking the habit. It probably wasn't easy