I'm not going to foster a better relationship between my in-laws and my son
Is there an unresolved issue between your in-laws and your son?
Yes, there are some history issues
I understand there have been some issues with your in laws in the past. You don't have to force a relationship between them and your son if you don't want to. Let your son spend time with people who want to be in his life instead, and this will not harm either party.
I'm going to try to fix things.
If the issue seems to be persistent, then I think you should go with your first instinct and not try to hard to fix this relationship.  Your in laws are practically strangers to your son, so you shouldn't be too rash in trying to force a fruitful relationship.  If something good can form, it will do on its own.