Hello, I'd like to help you with your question.
One option you can try is to ask him what he wants to call you. He is old enough to have some ideas of what he might like. If he gets stuck, make some suggestions. Instead of Dad, your husband could be called Pop for example. Or instead of Mom, you could be Ma, or a derivative of mom. Sometimes a nickname works instead of Mom and Dad. Or you could use a derivative of Aunt and Uncle. Some kids even call their parents by their first names, but you need to decide how comfortable you are with that idea.
The idea is to make the decision together so everyone feels comfortable. Your son will feel more a part of your family if he is included in the decision.
I hope this has helped you,Kate