S.,
It's a great step to solicit advice. But, my view on something so incredibly huge as building the internalized self-esteem of a child at such an impressionable young age is - you need to go talk with someone and work with someone who A) has qualified expertise and B) has NO STAKE in your life.
I say "B" because it's easy for others who have some kind of emotional tie to you - friends who appreciate your needs and relationship with your fiance, etc. - to NOT speak up about the child, first and foremost. That's just a fact of life. People don't always want to tell you what you don't want to hear. Now, I have no stake so...:-)
If your fiance is as fabulous as you describe, an 'outsider' like me might say, he should easily be expected to be able wait much longer than a year, if necessary, for the positive development of your son.
In my view, children this age don't understand how it's GOOD for them to not have their biological father in the picture. They process things on a base, human level of feeling - and he's got to be feeling abandonment, loss and an internalized sense that it was his fault - something wrong / bad with him that this most primary, key figure in his life would leave. It's a very common reaction in a child to divorce, let alone this kind of total abandonment (but don't take my opinion on this- go see an expert).
We've all see how kids long for and love parents who are drug addicts, neglectful, whatever. It's one of the most base, human needs / desires. Personally, I believe it cannot be filled by a new match and your just showing him that YOU are not going anywhere - it can only be filled in time and with the absolute, solitary dedication to HIS needs, long before yours / your fiance's.
S., I hope you'll forgive if any of what I've said is abrupt - the fact is, I am well accustomed to speaking so directly to girlfriends but not to strangers. But the mom in me says, you are this little boy's world. YOU. And the choices you make have HUGE impact on how he develops - so, in my view, it's worth taking time, baby steps and not jumping into a new marriage just yet - but, rather, seeing someone and working with your child's needs, first. Only when he is in a good, steady, secure place in himself and can accept change in stride would I, personally, then consider my own needs, as a woman.
I know, because of your reaching out for advice, you have a deep, deep love for your child. If this hits you in your heart, it will likely be because it rings true as a good direction to take - because you want what's best for your child more than you want what's best for you. Again, please forgive if I've said anything out of line - if it were me, I know it would be hard to hear anything that doesn't fit my desire to re-marry or whatever - but... I'd step up to that plate and go see an expert and take care of issue #1 first. Not only would it help my son, but it would be in the best interest of the new husband and the step son, toward building a calm, cohesive family unit. Time is on your side if love is as deep as you believe.
Okay - I wish you the BEST for your son, for you, for the step-son and for your relationship, too. May it all work out in the best interest of everyone!
Warmest Regards, T. B