At that age, the class is really as much about you getting to meet other moms and socialize as it is about stimulating him. I went to Gymboree for a couple of months when my son was 2-4 months old. Some days he'd stay awake and look around at the other babies, other days he'd doze and I'd sit in the circle and rock him and sing. You'll learn the songs and be able to repeat them at home.
They also have a time when they ask an 'icebreaker' question and go around the circle to the moms -- something about parenting, but it opens up to a nice little discussion for five minutes or so.
Sure, some days were better than others. There were times I sat in the circle and nursed him, and that was cool too -- it's what he needed, and there were all other small babies there, so there's not a rigid setup -- more go-with-the-flow.
At least that's what I experienced in my Gymboree class.
Unfortunately I really didn't click with the moms in my group. I actually felt like they were giving me a hard time for 'still' nursing (at 3 months!) and slinging Joey... so I when it ended I wound up finding a different class to go to. Still, if I had felt more at ease around the other women, it would've been great.