My son is nonverbal due to developmental delays. How can he participate during circle time when the other children sing songs? 

The caregiver can provide instruments as an alternative for children to play who are nonverbal or they can move their body to the rhythm of the music/song.  Another adaptation would be to provide props that relate to the songs (i.e. stars on sticks for Twinkle, Twinkle Little Star) to keep children engaged in the activity.  Each child should be able to choose how, or if they want to participate.