I guess, as a kid who was adopted by my mom's second husband--who was on the scene even before I was born-- that I am surprised how many people suggest telling a child as early as possible that they have a second father.
I'm coming at this not just from my personal experience, but from an understanding of a child's needs. This is from Maslow's Heirarchy of Needs (worth googling, and the foundation for much of our thinking around early childhood psychology): The very first and foremost is to have one's physical needs met; second is a sense that the world is a safe place; and third (not the last, by the way) is to feel a sense of belonging. Being a "staircase", this third step is a requirement for other 'higher' steps: "being love" (being loved just because the child exists); Compentency, and self-esteem. A lot of important, big stuff hinges on that sense of belonging. And this needs to be as straightforward and simple as possible for your young son, for a long time.
We adults often base our ideas of what our children can understand on our own experiences, and what *we* think a child can understand. For a young child to be told that they have a 'real father' who they will never see is incredibly confusing and hurtful. They do not have the comprehension to cognitively understand this, and may feel terrible that their 'daddy' doesn't want to see them. It would be a horrible thing to put on a young child, in my opinion.
I will speak from my own experience here and say that I figured out something was up when I was 9 or so. (My adoptive father and his family are Filipino; I am as caucasian as they come. I was bound to ask.) It was very good that I'd had all those years to bond with my 'growing-up' dad before receiving this news. It would have been better if my parents had decided to get some counseling for me, as they were divorced and mom was on hubby #3 by this time. All this to say, I was glad my daddy was allowed to be my daddy until I began asking questions.
And children *do* ask questions when they are ready to receive the information/answers they might get.
Your future husband, too, deserves to have his say in this. He is being more of a man than most men would in this situation, and really stepping up to the plate. Consider his feelings in this matter. I would honor his request to let him be Landon's daddy, with no strings for now.
As to when to tell him; I think this is a great discussion to have with a counselor when your son is older. Kids often need help to absorb this information and move through the feelings that may accompany them. I remember myself at that age, and I really needed a lot of support. You will likely benefit from support too.
If you wait until your child is older and cognitively, as well as emotionally, able to really understand all of the pieces of this: that this isn't his fault that his daddy has rejected and abandoned him (because these are very real, hard feelings, no matter if your present daddy is wonderful), that this ISN'T personal (although he might feel at times it was) and that your fiance was not just adopting him along with marrying you, but that he so much wanted to be his daddy-- you are going to give your child the better advantage of having built a strong 'staircase' to support him when a lot of his feelings about his own self are bound to feel challenged. And as I said again, my heart goes out to Val, because he's really doing the best he possibly can within the situation. Waiting will save you all a lot of heartache and let you become a family of your own first, unfettered by the ghost of the father who could care less. He'll get his time, too. But let that be a long, long time from now.
Best,
H.