It sounds like you are a very caring and supportive partner to your husband, and you care about his son as well. That is wonderful. You are also taking the mom's feelings into consideration, and that is very honorable, too.
It sounds like the son wants his father to be a part of his life, and is reaching out to his father. It would be important for your husband to be consistent in his attentions to his son now, whether he wants to limit it or have open contact - it's really what he and his son are comfortable with (as much as it is good to keep the mom's feelings in consideration, the ultimate decision for contact *is* up to the son and your husband, with you and the mom allowing space for the father-son bond to develop as they need it to).
One of my good friends told me about his relationship to his dad growing up, and said that his biological father disappointed him a lot, and often didn't keep his word. I think it affected him, and made him lose trust in his biological father (and there wasn't much to begin with, for other significant reasons). Another friend told me that he could see through what his father was doing, and did not appreciate his behavior when he was a kid, but they have since reconciled, and he accepts his father as he is... I guess what I am trying to say is that fatherly love is important to a child, and actions do speak louder than words. So, if your husband is jumping into being a part of his son's life now, he needs to always keep his word, and be there for his son. If he is worried about his ex, he needs to get over that and not put his feelings about these difficulties over the feelings of his son. Sounds like he is starting to be there for his son, and you are a very supportive partner. Best wishes to all. xo.