I'm sorry you and dh are going through this. When a marriage hits a bumpy patch, it's hard to think about anything else.
Counseling is definetly a must in this situation. He's been seriously considering seperation, without telling you why, beyond needing to work things out in his head. That's a huge red flag. People that won't talk about the things that are bothering them, using the 'work it out for myself first' line of reasoning are not hearing other's POVs. The only conclusion they can come to is what they've worked out, and chances are it will be a self-serving solution.
I would calmly tell him that he has to open up to you, because it's now or never time. If he feels more comfortable doing it in counseling, where there's a 'referee', then make the appointment [b]now[/b[. If he does open up and tell you what's bothering him, be prepared to hear things about yourself that you won't like at all. Don't go on the defensive, and don't argue with him. He'll just clam up again. When he's gone through his list, ask him what you can do to make things better, within reason.