We go to Lifetime Fitness, but I'm sure their childcares are pretty similar. The most important thing is that you've checked out the facility to ensure it's safe for young ones.
Be prepared to have some crying and clinging when you first start going. My son was an infant when I first started taking him to Lifetime, so I figured he'd be used to being left there for two hours everyday (I always shower and get fully ready after my workout). However, when he hit the toddler stage, he started with being upset when I left him. Of course, he was always fine by the time I picked him up b/c they have so many fun things for the kids. And, that stage didn't last long.
Also, as one mom said, your son probably will get sick from being exposed to other kids at the center. Just know that your kids are going to go through that at some point in their lives. If they don't build up their immune system by being exposed to colds, etc. when they're young, it'll just happen when they go to school. My son was sick for about the first month that we started going to Lifetime, but after that, he's been fine. In fact, he rarely gets sick at all now. And, since my daughter had been at a daycare for a while, she had already built up that immunity and never got sick from being at the gym.
Kids are tougher than we usually give them credit for. There may be some adjustment time, but I'm sure your son will do great.