Need advice about my son... people in this situation?

my son will be 2 on sunday... his biological father has never been in his life... and only "paid" child support because the state filled for it, (thats a different story though) he only paid the first few months after it was ordered last summer then stopped... they started taking it out of his check at my request about 2 months ago and it caught him up... he is only ordered to pay $90 a month. my husband is his daddy and the only person my son has ever known. my husband changes his diaper, plays with him, financially supports him, and loves him dearly. we also have a 2 month old son, all 3 of us have the same last name while my 2 year old has his fathers last name. he has never came around, called, or ask people we mutually know about him (to my knowledge). he recently took off with his wife and girlfriend and the girlfriends kids (dont ask cause idk lol) and moved to texas ((we live in tennessee and he did too)) we have talked to a lawyer and have a good case for abandonment so that my husband can adopt him. the only problem is, im worried about what is best for him. im thinking when he turns 13 and his old enough to understand i can explain to him that his father wasnt mature enough to take care of him but loves him very much, and my husband is his daddy and always has been. but i know this will still confuse him, do you have any experience with this kind of situation... can you offer me any advice?