Your lawyer should be able to advise you in the changing of surname.
Why do you need to change your sonís surname? If you are afraid that your son will not benefit anything from your future husband, rest assured. Your future husband has the responsibilities to provide for him for sure (and itís legal). If you are changing his surname because of personal reasons, I feel sad for your son and your ex. You canít wipe your past. Give a chance to your future husband to bring him up without any short changes. When your son is old enough, he will knows who to be filial to. If your childís surname reminds you of your past, thatís for you to deal with. dontít make your son bear it. God bless.