I am so sorry about your fertility woes. That is heartbreaking. Only you know how strong your marriage is. How do you think your husband would react to this man and his offer to father a child? What is it about this man that he does not like? Is it something that would cloud his judgment and make him say no without a second thought?
If I were you, I would thank him for the offer and talk to your DH. Considering he has been there for all through ups and downs, he may not be alright with this guy's offer. On the other hand he might because a baby is the one thing you want but cannot seem to have. Then again, it might make him feel inadequate. Only waybto find outfit to talk to him or get a feel for it by presenting a hypothetical scenario. Have you and your husband considered a surrogate to give your body a break? I know it is not the same as carrying a child, but fertility treatments are already emotionally taxing and hard. Your marriage seems strained, and I am sure the stress is not helping your treatments.
There is also nothing wrong with seeking more opinions. Someone may have the answer as to why it has failed. Sending hugs your way.