I agree with some of the others, what he did (having a one night stand while taken) is something that shouldn't be taken to harshly or held against him at this point.

OH. Except the part where he completely lied about it for two years. Kept the lie up. For two years. That's NOT ok.

The action he lied about is something that didn't need to even be lied about for so long and is definitely a blow to the trust factor between you two.