A 2yo isn't going to understand the concept of "BF", so just introduce him as (his name). You don't have to explain the relationship right now. After they get to know eachother better, you can give BF "babysitting priviledges". Hanging out with BF while you go to the store will seem perfectly normal to your son. As their relationship solidifies, babysitting can turn into "sleep overs". Eventually, your son will be old enough to grasp the concept, there may even be a point in time when he exclaims, "I wish you were my Dad."
Talk to BF about how you think this should be handled and ask about his thoughts on the matter. Make sure that your decision puts your son's welfare first. Don't push BF on your son, he doesn't have to rush into the guys arms and yell, "Daddy!" Let him open up to this man in his own way.
It's great that BF wants to be part of his life. Right now that is going to mean that BF comes over for dinner. Goes to the park with you and your son. And generally comes around every once in a while. Including BF in things will help your son in the long run.
I should add that you also need to be considerate of your husband and discuss the situation with him. If he knows that your son is BF's son, try to help him understand that your son thinks of him as "Daddy" and you aren't trying to take that relationship away from him.