I seriously doubt that there is one person, or group of persons on this thread who is qualified to evaluate Nickey's situation and then to pass judgement on it or on Nickey's boyfriend.
Each person is different, therefore each relationship is different. You all might be right in that if he's done it once he will do it again. You also might be totally wrong. You just don't have enough information to make that evaluation.
Some guys are terrified of committment and will do something stupid like this so that they don't have to face making a decision on the issue in terms of long term committment.
If I were you Nickey and I loved this guy still, I would take him back on a provisional basis. Only I wouldn't let him know it was provisional; I'd keep that to myself. I'd go about my relationship with him just like nothing ever happened. If you brow-beat him about it, if you put him on guilt trips, if you won't let it go, you'll never get a fair idea of what he's really about.
If you love him enough to take him back without a word about his mistake and closely watch his behavior over a period of a year or so, you might find out that he's vastly worth more than you ever thought. And don't let him keep on apologizing for his behavior. Let him get it out fully one time and then tell him you don't want to hear anything about it. You two can't develop a decent and worthwhile "now" if you continue to let "yesterday" look over your shoulder. Same for tomorrow. You can't plan a happy tomorrow if someone is always wallowing around in "The Mistake." I've known plenty of people who have made such a mistake, who have handled it sort of like I suggest here and who have been able to put it aside forever and get on with their marriages like nothing happened. Do you love him enough to give that a try? Even if you only "think" you do, then likely you should, or you're likely to go through your life wondering what would have happened. And you don't want that. That constant, day in and day out nagging question: Should I have at least tried with him one more time to see if it might have worked.
Go slow, NIckey. You might only have this one chance to see if you've got something worth working for here. And you don't want to live a life of regret if you shoot him down now without a chance to prove himself.
Think about it. I'm sure you'll do the right thing.