How do I convince him I am done? 

There is always hope. I am a firm believer that everything happens for a reason. And even though things seem bad now and you can't figure out the reason it will come to you when the time is right. Whenever things take a turn down a dark path I try to remind myself of that. It kind of keeps me going giving me hope that maybe around the next corner the light will shine a little brighter.

The way we handle our problems define who we are.  You husband has demonstrated his character.  You must act accordingly by first seeing an attorney to ensure you and your son are protected.  A decision is very hard to arrive at sometimes, and you have decided.  Nothing will be done, as he will drag his feet toward any divorce.  So the job is left to you to take action, start by starting. 

Keep your son out of the loop, don't trash your husband to him, don't make your problems his problems.  He will come to his own conclusions in time.

It's obvious your husband has not earned your trust, and he was careless to have done what he has done.  You were treated without respect and your son has a poor role model in him.  You must realize that it is just as hard to divorce as it is to reconcile.  Act prudently and not rashly.

When you're done with a marriage there is nothing to do but see your lawyer who will file a petition with the courts to dissolve the marriage.  Your spouse will get served divorce papers and everything will be handled by your attorney.  You don't need to have the approval of your spouse or anything.  Just do it. 

To send the message to your spouse, move out of the bedroom.  Have no sexual contact.  Begin living your life separate and apart under the same roof until one of you is awarded the house to live in and the other must leave.  End of story.

If your truely done, there is no need for convincing. Just do exactly what you say your going to do. I know in my situation I have been saying for year that I'm done. But he has grown to know over the years that if he's "good" I am willing to keep trying until the next big screw up. Now it's like a poker game he's always calling my bluff.

Thank you, that was very good advice.  I am doing all the things you said, there is no hope, my well is dry.

If you are truly done with the marriage, then you make sure you back your words up with action.  If you can't afford to move out of the house, then you move to a different part of the house (spare room, basement, couch, etc.).  In extreme cases, you just don't be home there for most of the day when he's home.  Limit conversations to the essentials...bills, kids, etc.  If he tries to steer the conversations towards forgiveness and reconcilliation, no matter how animated and upset he gets, you keep your cool and tell him, calmly, that you are done with the marriage and are only waiting for x, y, and z items beofre you file.  Start doing things APART from him...if you attend mutual friends/family functions, go seperately and leave seperately from him.

The point is, even if he is in denial and thinks the marriage can be saved, he may try to beg and plead for you back.  You stay strong and back all of your words up with action.  If all you're doing is waiting on selling the house, then you work on fixing what needs to be fixed on the house (assuming he's dragging his feet on that), and you call the realtor/list it yourself, whatever...but YOU do the work if he's dragging his feet.  You control YOUR spending to get those bills paid off.  Start doing research on divorce and what you need to do to get the ball rolling on that.  And whatever you do, do NOT send him mixed signals.  The worst thing you can possibly do to him is give him false hope that the marriage can be saved, so try to avoid a lot of "Well, if X happens, then there's a possiblility that Y can happen."

I hope that helps...good luck and keep us posted!!!