You can call your son by whatever surname you want.
A relative of mine was in a similar situation - she was divorced and then remarried when her daughter was 2 years old. The daughter was known by her stepfathers surname from then on.
When she came to need her first passport at the age of 16, someone in authority (I think it was the family GP) wrote a supporting letter saying that she had used the surname Bloggs for 14 years.
Her passport is in her stepfathers surname, as are all her legal docs. She is 23 now and has never had any problems over her surname.
Your hubby could adopt your son, or you could change his name by deed poll, but frankly, it is a waste of money as you don't need to.