I just wanted to say: whatever you do, do NOT take him to Adventure Kids Playcare in Plano! They have children running wild all over the place (TOGETHER not separated no matter what they tell you!) with minimal (if any) supervision. I know this from a very good inside source.
Your son would be lost in there! They have kids from 18 months to 12 years old running all over the place. They will tell you they have a "toddler" room, but it is used just like the rest of the place - kids coming in and out.
There is also someone on here who is constantly praising Adventure kids and talking about how much she loves it and how her boys love it there. I found out that she works there and is good friends with the owner.
As for socializing, it sounds like you are doing the right thing - some kids are shy; i bet as time progresses he will get better. Otherwise, I would consider enrolling him part time in a preschool (or early preschool). This not only helps with social skills, but it also gives them a head start on manners at school, and how to act in a setting other than at home or with mom and dad or friends. Just a thought...
Good luck!