Totally normal lah, don't worry! At 2, a lot of kids are still shy or overwhelmed by the stimulation, especially in a group setting, some of my mum friends' toddlers also prefer to observe je dari tepi dulu before they're ready to jump in. The music class is still beneficial because dia absorbing everything even if he's quiet, and once he feels more confident nanti dia akan naturally start participating. Just let him go at his own pace, no pressure!