I think exposure is the key. My son is an only child, almost 2, and has been going to Gymboree since he was about 18 mos. He does "play" and "art" 2 days/wk @ 1 hr each, and sometimes "open play," which is unstructured with no teacher. I wish I had started sooner. He has really blossomed socially. He was not as shy, perhaps as you have described your son, but definitely reticent. There is a ~2 yr old boy in his play group, however that is as you described your son, maybe worse. He would cry throughout nearly every class. His grandmother brought him and she was very kind and gentle, never pushy with him. He is like a different kid now. I still wouldn't describe him as overly gregarious, but he is enjoying his classess, happy, and playing like majority of the other kids. This is just a guess, but I think a MAJOR factor may have been the fact that his grandmother was very friendly and talkative with the other moms and caregivers. I am sure he picked up on her calm and over time just realized this was his normal routine and not only safe and OK, but fun.
So at the risk of reading too much into your post, I wouldn't underestimate the power of any emotions you may be projecting. Does a little voice crop up around strangers that says "Uh oh, he's going to freak out I better get him out of here"? Do his reactions cause any anxiety or self consciousness on your part?. We all want our kids to be happy functioning people so it can be a little embarrassing, if we let it. Only the problem is, he will pick up on emotions like these, and it will make the situation worse.
I am inherently shy so I know it can be tough, but I would encourage you to put yourself out there, align yourself with other moms, and model the behaviour you would like to see in your son (being relaxed, talkative, joking around, etc.) With routine, and exposure to other children and their (understanding) mothers, he should come around. He may never become OVERLY friendly, but at the very least he should be comfortable in the situation.
Also, although I don't necessarily advocate Gymboree specifically, this or something similar, might be a good prep for him...a "pre pre-school"! They introduce structure and routine, have teachers, and are fun, interactive, and low-key, expectation-wise....Unfortunately, they cost money ;)
A play group would be very good as well. It may not have the same level of structure, but it would get him into a routine and get him exposed to other kids.
I think you will be surprised the difference a year can make. Best of luck to you and your little boy!
K.
PS...It's entirely possible I have misread your situation. You may be the life of the party for all I know!..LOL..
I do apologize if so.