Most children only need one or two good friends during the school years so don’t worry about your son having a lot of friends. And- just because he doesn’t have a close friend now doesn’t mean that he won’t in the near future. I know that you worry about him being lonely but many single children don’t feel this way.
As far as how much time he can be alone after school, many children do get lonely coming home after school alone if this happens regularly. They aren’t necessarily lonely for friends, but for their parents. I do think that it would be a good idea to have someone at home with him after school if you can. I’m not sure if a boy is a good idea (unless you know him very well) only because adolescent boys with time on their hands and nothing to do can be trouble. I would look for another mother or grandmother who would be able and drive him places.
In the meantime, do the best that you can do and know that it is enough. You sound like a very conscientious mother and I’m sure that your sons feels that you care and love him. Make sure that when you are home with him that you and his father spend good time with him. As I said, many, many children grow up as only children and are happy and fulfilled. In fact, in some ways they are luckier than other children because they get more of their parents’ time and attention.