Take this from someone who has been cheated on: you will never trust him 100% ever again. Ever. It will take a toll on your relationship over time.
Every single one of my girl friends have been through this, and it always turns out the same. First of all, once a cheater, always a cheater. There is almost never an exception to that rule. Secondly, the trust never returns. The best thing to do, as hard as it may be, and as sincere as he may seem, is to walk away. Find someone who respects you enough to never hurt you like that or betray your trust.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi