Legal Question in Family Law in California

It does not depend on who has physical custody. It is common that physical custody be awarded to one parent with visitation to the other, and then legal custody is "joint." Unless you have sole legal custody, which is usually reserved for when the other parent is deemed unfit due to substance abuse, violence, or criminal activity, etc., the father was able to legally enroll your son. On the other hand, unless it would breach the contract, you can go in and cancel it. That is why most agencies require both parents signatures. Then it's up to one or both of you to go into court and sort it out, unless you both can decide to start appropriately co-parenting and work it out.