Lou - have logged onto this a bit late. Am thinking of you and really hope things work out.
I can understand that you have to make the trip for your own sanity and wellbeing, and personally think you're absolutely right to go. I liked the point you made about your dh spending some time with the kids and the washing machine (my dh always appreciates me much more after a few days spent with the children and without me!)
I wouldn't underestimate how much reassurance he may secretly be needing though. I think someone else made the point that it is great (and quite unusual) for him to have so much insight into his own state of mind at the moment, and to come clean about it. Your going away, however much for understandable reasons, is obviously worrying him a lot.
Although it might seem like overkill, could you perhaps arrange for one of your local friends (who sound such a wonderful bunch - really supportive!), to pop letters / cards you've written in advance into the post every few days? That way he'd get regular tangible reminders of your love and support to keep him going.
I was really struck by how you said his mother didn't reply to his letters, so perhaps by sending him regular written reminders, this would show him that you are different from her.
It may be a bit naff, but it would be symbolic of the fact that you haven't given up on him, that you're thinking of him even when you're not there, and that you're going to come back. Perhaps they could even be written in a countdown sort of way - you know, 10 days to go, 8 days to go etc.?
I find what you've said about his mother so distressing, but it seems that it's quite common. I know that a friend of mine had a similar situation with her father. It was awful to watch - every time they had contact she would build her hopes up, and then I'd have to hear her making excuses for him as to why he'd not been able to do this or that, and you could just feel her hurt. In the end, she cut off relations with him for a few years, until in the end, he made contact when she had kids. She was very calm and pleasant about it, but basically told him that she wasn't prepared to have a relationship that was so one-sided. By being so adult about it, he was forced to be an adult too (good example of transactional analysis situation, if you've ever come across this in your counselling experiences).
I think I may be local to you, since I'm also a Waverley person. Please let me know if there is anything I can do to support you or help you. I know we don't know oneanother from Adam, but the MN world is a small one (and a friendly one) isn't it?!