Rather than being insistent that your son joins a club, explore with him reasons that he might enjoy participating in one. For example, the computer club could help him pick up new skills and meet others that share his interest. Also, your son might be less hesitant to join any club if he had a friend to do it with him so he would know at least one club member.
Not everyone is a joiner, and there are other ways to participate in high school activities than after-school clubs. Your son could go to school events from football games to plays with his friends, or he could participate in intramural sports or music activities. If your son is a particularly good student in a subject, he could become a volunteer tutor or student assistant in that class.
It is definitely a concern that your son is coming home and not having any interaction with other students after school. Encourage him to spend time several days a week with friends rather than in solitary activities after school. High school can be overwhelming at first. If your son has a few good friends, he should soon find ways to feel more comfortable in this new environment.