Jo,my eldest son is from a previous relationship, we have been with my DH since he was 2.5 years old and even though technically he is his step son he would never ever refer to him that way as he loves and cares for him just the same as our other boys.
DS1 calls DH dad and his biological father by his name.( THATS ANOTHER STORY FOR ANOTHER TIME)
DH has been asked if DS1 was his and his reply was yes, he is my son.
It is hard when people ask questions like that but you don't have to explain your situation to anyone,we don't.
Take care
Michelle