What can I do if my son was taken away from his mother by CPS and placed in a mental health institution due to behavior?

First things first: Retain an experienced lawyer in your locality who has dealt with state commitments and juvenile matters. Were you given notice of the hearing which led to the commitment? Most states recognize your right to be present. If you got notice, did you attend? Did you object? Did you propose an alternative placement (perhaps with you)? Your lawyer might have a number of alternative modes of proceeding, which could include a motion to reconsider; an appeal (not often successful), a petition for custody, even a writ of habeas corpus. But you need professional assistance, and the sooner the better.