It could be that he feels in direct competition with your grandchild for his wife's affections. Since John's problems still exist, it seems obvious that they stopped counseling too soon. If they truly want to work on their relationship, I would encourage this couple to seek additional professional help. Whatever you try, you will be caught in the middle of conflicting loyalties and emotions. I don't think your son is a jerk, but he does have problems that can be addressed in counseling.
Chronic infidelity can also be the unfortunate coping mechanism of an avoidant personality.
These individuals avoid interpersonal intimacy and commitment as well as sincere and deep communication. When problems arise in their marriages, an avoidant person may turn to a third party to manage his or her anxieties. The reasons for John's continued infidelity need to be explored and a plan of treatment made based on this assessment. As loving parents, that can be devastating for you - especially if you did your best to rear your son according to solid moral principles. You and your husband may be tempted to blame yourselves or even turn on each other.