I understand OP that this may be unpleasant to hear but first of all this was not a little mistake, yes everyone does make mistakes but some are bigger than others, sleeping with someone else is not a small thing, i mean the number of cheaters there are out there is suprising considering as far as im concerned its really not that difficult to not sleep with other people.
anyway the point that im making is that you should just move on and let this poor guy do the same as there is a big difference with forgiving and forgetting and even if this guy one day forgives you this mistake means that the trust will likely never fully return and as result any future realationship with you two is likely to be doomed from the outset. if you cant even trust the person your in a relationship with why bother.