How do I help my son who does not make friends easily to make friends?
Making friends takes time. It took us several years to develop close friendships with other homeschoolers. Field trips are a good place to see other kids your son's age. The best way to encourage friendship is to invite some of those kids over. Home turf seems to be easier ground for someone who is reluctant in friendship.
The best way I have been able to help my son who sometimes struggles with social situations is by making an effort to ensure we have play dates. I found it helpful to keep them short at first, and sometimes to have a neutral meeting place for the first meeting like a park or a McDonald's. But the biggest thing is making the effort to ensure they get play dates regularly.
Arrange play dates with another child so the experience is with one on one play. Or, you could sign up for a class that would interest the child, and where the class size would be small.