We have joint custody of our 14 year old son, and back in 2011, biofather changed sons last name without biomothers permission. Is there anything that I can do about this matter?

How did this name change occur? Generally it takes a court order to change a name legally. But a person can use any name they want as long as there is no intent to defraud.

It would be very difficult to get a court order without both parents permission. So you need to get your facts straight before determining what you have to do. This means determining if a LEGAL name change occurred. And if so, what court issued the order.