Sometimes this can happen and if he's been good since then I wouldn't worry. However, because you are asking the question it appears you have a gut instinct he's up to his old tricks. I believe in listening to gut instincts. Once a person cheats on another there is a trust issue and the relationship seldom will last the test of time.
Sit down with him and have a long talk with him. Find out who he did it with, how many times and for how long. Find out if he regrets what he did and if he might do it again. You have to also assess your feelings. Are you willing to take him back, or is it time to cut your losses and move on.
That is the worst thing a person could do ... break trust in a relationship. If he hasn't been giving you any problems since, then he could have simply made a mistake (we all do) and you have to ask yourself if he's worth the risk. Only you know thing young man and only you can decide if he's worth forgiving. Dump him. Once a cheater most times always a cheater. If you want to take that chance it is totally up to you though. Why did he cheat?
Uh breakup with him
Dump them
They both are bad. If your best friend cheats on you with your boyfriend, she is a NOT a loyal friend. And if your boyfriend cheats on you, DUMP HIM.
Are you physic?
dump him
yes but she cheats on him.
Ask him.
You torture him.
YES
shoot him in the head
it's still cheating.
Time to say goodby to your boyfriend and probably your bff, too. Why would she go out with your boyfriend if she was your best friend?
It means he doesn't really care about you
fool me once, shame on you. fool me twice, shame on me.