What should a child call their stepdad?

My 4 year old calls my husband dad. He started calling him dad all on his own. He knows his real and who he is, but doesn't have much of a relationship with him. Since day one it's been me and my son, by his own choice not my doing. His real dad may see him once a month if that and not on a regular basis. He doesn't pay child support and we don't ask. He does have our son on his health insurance and pays for half of daycare most of the time, but that is it.