It depends I think.
In my case, I was introduced to two out of the three children about 4-5 months into our relationship. Their mother was already re-married and had a new baby. Both children liked me immediately upon meeting me and that continued until the third (eldest) was introduced. The eldest and her father didn’t have the best of relationships and she had an immense sense of loyalty to her mother, so sadly things then slid rapidly from there. I did not know about the instability in their relationship and the reasons why until I was very much embedded. It was very difficult.
Some children will often feel a sense of misplaced loyalty to one parent if they allow themselves to like a parent’s new partner. Sometimes one of the parents is still very bitter or resentful regards the separation and sometimes the child or children will directly or indirectly understand this and thus they will start taking sides.
If your son’s strong feelings have come about due to the above, then he may never come round to accepting your new partner regardless of how wonderful he is to you or them. Or he may do.
There are too many variables in order to make a diagnosis. So I would prefer to reframe this for you. Instead of hoping and wishing for a time/date as to when your son will have a change of heart, I would take more of a view on how you will navigate what may be a very rugged emotional landscape to come.
Your children do not necessarily have to like your new partner, but they ideally should be civil and respectful towards him just as they would be to anyone else. This I cannot emphasise enough.
I would not go in there with expectations of a new harmonious step-family. Step parenting can be pretty tough and the terrain is very different. So I would advise that you approach it differently. It is ok to introduce them to the new partner but I would start with small amounts of time like lunch and the park. I would also advise that the meetings are in neutral places so that everyone gets time to adjust. However, if after sometime of doing this and your son still has his heels firmly dug in, then you will likely come to a place where you’ll need to make an executive decision.
Personally, I would continue on in the relationship if three out of four of you are happy. But it’s your life and your decisions.
Good luck