You could still try to talk to the son (and just don't have a negative expectation of his reaction) If you do, he may sense that and it would lead to a self fulfilling prophecy. Of course, you do not have control of anyone's reactions only of your own.
If you feel that others are resentful, that is their issue. You are just caught up in the middle. It would be most helpful if your partner addresses this issue with his kids. You ought not to be the mediator or the door mat. His decision impacts them and he has to address it.
If you decide to speak to the son, perhaps you would want to do it some place out of the house - coffee shop, in the park, etc.
Outside counseling may be quite helpful (to help you figure out what is making you hold onto it and to help in letting go) Your husband seems supportive (addressing this when you got married and is not years later the issue still hangs over your head)
As far as befriending the daughter, you can remain civil and do not have to trust anyone with alternative motives.