Hi hun, I did and thought the same as you. Gave Ellie his surname thinking one day I would have the same surname and we were happy etc...... I have just got her name changed to mine though
I made an appointment with a solicitor who sent a letter to fob that he had to sign and send back, it took 3weeks as he was dragging his feet but we got there in the end. Im guessing it will be the same process to change it to a double barreled surname, if fob is on the birth cert he has 50/50 say so, so he will have to be contacted. Have you spoken to fob about changing lo's surname?