I have a son who has some social skills issues, too. We have been working on them with a psychologist and a social skills group for a couple of years. The best book I have read on the topic is "8 Keys to Raising the Quirky Kid" by Mark Bowers. It is an incredible resource and is the only book I've read that provides detailed advice about how to scaffold various social skills. I highly recommend it for you.
I have also bought and read some books aimed at kids. If your son is open to reading them, the following books might be helpful: "How to Make and Keep Friends" by Nadine Briggs and "Social Rules for Kids" by Susan Diamond.
My son, who is now six, participated in a social skills play group for over a year and that helped a bit. I saw the most dramatic improvement in his social skills, however, when he started attending a school where he found a friend who was also very smart, a bit quirky and interested in the same things. For kids who struggle with social skills, lots of practice (combined with some direct instruction and modeling) can make a huge difference. The Bowers book stresses this and I found it to be true. That book was a lifesaver for us and I think you will find what you need in it. Good luck!
