Tell him the truth. Tell him what you just told us and explain to him that you are laying everything on the table. Tell him you expect complete honesty; if this is a loving relationship he should respect your wishes.
Be completely forthright with each other. If he does come clean and admits to cheating, then you will have to do some soul searching because your "trust" level will have to slowly develop again with him and vice versa. In my opinion, this way is better than not knowing and being suspicious and dealing with all the mental conflict and ramifications of being silent.
Best of luck to you girl.
*hugs*