Someone asked to me answer this question so I will
from what information is here, I will answer to the best of my knowledge...facts left out are child’s age, are you married to this man or just in a relationship? Does this child have a drug, alcohol, or mental problem?
Without all of that information I will answer it this way. The father and the son had the relationship prior to you, meaning that their relationship came first before you, depending on the child’s age, it could have been quite a while that they were in the relationship before you, developing their way of communicating and interacting with each other. At some point you came into the picture and have changed the dynamic between the father and his son.
I am not sure if you have children or not but, the love that parents have for children will almost become a part of them, they are a reflection of the dad. If the dad caused the separation of the family and now or in the past has any guilt around it he may have spoiled his child materially tiring to make up for it, or he may have let his son get away with bad behavior because of it?
The point being is that with all step parenting situations, the person getting involved has to know that they are not first, and did not come first. The child is first and even when that kids gets older in many instance the parent will still come to their sides first. So the real question is how you are going to cope with it and do you want to stay in the relationship, expecting him to change meaning the father is very unlikely and I am sure that he knows how you already feel.