Legal Question in Family Law in California

the only legal way is to have someone else adopt the child.

Unless he agrees, or he commits criminal abuse or neglect of the child, you can't. Quite frankly it would be very short sighted of you to want that anyway. As long as he has rights, he also has support obligations. Your child is entitled to support from him and y should collect it. Your child should also have a father. 18 years is a LONG time. Who knows what that man will be like when your child wants or needs him in 5 or 10 or 15 years.