My son says he does not want to get to know his father, as a mother what should I do?

My ex boyfriend who is the father of my ten year old has did very little to get to know his son. My husband of 8 years is the only father my son has really known. His real father has seen him twice and talked to him a few times on the phone. He uses the excuse that he lives in Australia and my son and I live in the States as a excuse. I asked him a few month's ago if my husband could adopt my son since he was not interested in being a dad. He said No and even told me that he wants to make more of an effort. He is coming to see him next week since he is off of school for the summer. The problem is I told my son that his dad wants to spend time with him and is coming next week and my son said he did not care and he did not want to get to know him. I am not sure what to do about this. As his mother should I encourage him to open up to his dad? Or should I let my son decide this on his own? I am weary about encouraging him in case he does end up hurting my son, if he did that I would be partly responsible because I did encourage him. My husband will not give me advice on this and has not said one word about this situation. He did say if my ex hurts my son again he will tear him to shreds. Does anyone have some advice, I know its my decision but I am really confuse on what to do about this. Thanks!