I LOVED our Music Together classes. And I'll be honest...I became very frustrated when my son preferred to play with the window shades in the room than sit with me and sing the songs. But then I saw that, once the kids got to toddler age, very few of them want to stay seated (and you really can't force them!) It's just part of the age.
Our instructor encouraged me not to be frustrated and to simply let my son move around as he wanted (the room was baby-proofed as best as possible). Forcing him to sit would have made him and me miserable -- and THAT would have been a waste of money. Her philosophy was that any positive exposure to music is a wonderful learning experience for the child. Now, in retrospect, I can see how much my son still loves hearing the music from those classes (Both kids still ask me to play the CDs in the car.) The class really had an impact on him -- and both kids remember and sing the songs.
Most of the class is spent with active play along to the music. You and your child may have a musical instrument to play, may have a scarf to throw in the air, may have a dance, may learn a game to the song, etc. These are the parts of class that even the highly active kids (like my son) were very involved.
For me, the best part of the class was the music. I'm not a huge fan of "silly" songs for kids. Music Together used real music from all over the world, as well as some American classics. My husband and I enjoy listening to the CDs and singing along with my kids when we are on road trips.
One last thought...the key to this class is singing along with the instructor. And your instructor will coach you: it doesn't matter that you don't have a professional singing voice (who does???) Your child loves hearing your voice and will be thrilled hearing you sing. It will encourage your child to use his own voice!
Sorry this was so long, but I hope this helps.
EDIT: This class is intended for the youngest of kids. Your son is the perfect age.