If your overweight teen boy doesn't seem interested in participating in a weight loss support group, it may be that the group is mainly comprised of girls, which may not address his concerns. After all, feelings about weight, shape and appearance affect the sexes differently.
And males communicate differently, too. Adolescent boys need a safe place to express themselves openly - using a common language.
If there are no male-based weight loss groups in your community, you might try approaching the school social worker to suggest one.