You can explain that there are different parts of being a dad. One part is providing the sperm to grow the baby in mums tummy. Another part of being the person who loved and cares for the child and brings them up.
In some families, it’s the same man who does both bits of being a dad. In others, like yours , it’s “Steve” who put the sperm in you to grow Ds and his dad who does the other part.
Please be measured in what you say about Steve. Remember he had a good side to him otherwise you wouldn’t have got past the first date. Some of these good qualities are in his son.
I say this not I repeat NOT to protect Steve. I say this for your son sake. His father is half of his genetics . He doesn’t get that now but he will in a few years. You don’t want him to think that half of him is evil.
When he is older, you will need to tell him more about Steve in an age appropriate way.
If you son asks to meet him, explain that you and he feel out when you were pregnant, that Steve went off as sadly he wasn’t ready to be a dad (assuming that’s true ) . That he wasn’t ready to be a a dad to any child, he didn’t make very good choices in his own life.
Say that he never met your son, or he would have seen what a very special child he is. And how very proud and happy his ( step) dad is so get to be the dad who cares for him every day.
It’s important he know his bio dad didn’t leave because he was a bad baby.
When he’s a little older, maybe 5-6, your son might express sadness that he doesn’t know Steve. Please don’t tell him not to feel like that, because he’s got his SD / you / whatever.
Tell him its ok to be sad, that what Steve did wasn’t fair and it was a bad decision, because if you make a baby you are supposed to care for it.
Don’t tell lies about why he didn’t stay eg he was sick, he had to work. That can make children think that you/ his SD might leave too.