Our son is very shy, too. He's definitely a follower, not a stronger personality that leads other kids in activities. My husband and I both have strong personalities (mine is the more dominant), and we embrace that our son is a different personality and try to encourage him as much as possible to be himself (he is 3.5 years).
I think all you can do is to continue to be acutely aware of him being the middle child, having a different personality, and making sure he doesn't get lost in the shuffle.
My only recommendation of what you can do differently than what you're doing now is to make sure you're out of the picture in some of the activities. We've been fortunate that our son has been in day care the past 2 years and has had exposure to different social situations in our absence and has had to work through them on his own. As much as I applaud you for engaging him in gymnastics and T-ball, your presence there may be his safety net.
He sounds like a wonderful child, and I hope he'll be very comfortable in his own skin as a different personality from his siblings. One thing we're trying to make a concerted effort to do is to have individual time with each of our children - one-on-one time with Mom and then Dad separately. We want to let them pick the activities and really get to know them for the individuals they are.
Good luck. It sounds like you're a wonderful mom to all 3 of your kids.