I know - it is really difficult. But look at it through his eyes. He hasn't been through the whole DBing process like you have. I'm sure he feels, like many kids, that your H cheated on HIM too. And he probably feels that your H no longer has the moral authority to tell him what to do. Add to that the normal alpha-male friction between fathers and sons this age anyway, and the step-parent factor - well, I can't say I blame him. Just because you forgave your H doesn't mean your son will - or that he has to.
And, after all, you are receiving the nice cards and flowery words from your H which make the reconciliation process easier - but what exactly has your H done to reconcile with your son?
I suspect the best approach would be to simply continue dating your H for now. Let S see your H being more responsible (and hopefully H will try to apologize to S for devastating his homelife at such a vulnerable age and for setting such a poor example for S). If S and H reconcile, then H can move in. If they don't, then S will be out of the house on his own in a year and a half, and you can move in with H then.
If your S is really doing all the right things in his life otherwise, and being responsible, I think you really need to respect his feelings on this. After all, he didn't choose any of this.
And if H backs out of the reconciliation because he doesn't have any empathy for the pain he has caused this child? Then he's not man enough for you anyway.
