Hello, another one here. Just over three months ago.
The reason he has done it is always going to be a pre-occupation for all of us, simply because it is out of character and unexpected.
But the reasons matter little - you first and foremost have to make practical arrangements to cope, get through the first awful weeks when the disbelief wears off and the horror kicks in.
My biggest regret was not telling anyone for two weeks. Like yours, my husband was a lovely man ordinarily. The sooner you tell people, the more able you are to cope with the practicalities, e.g. getting time away from the kids to just cry, or to get out and do something that's going to make you feel better, e.g. a jog or hair-do, a message: whatever works for you.
It is the kindness of strangers that will keep you going. People are amazing in how much they will help if you ask for it. Make sure you accept any help that will benefit you.
The trick is to try to keep yourself busy as much as possible - I know your head goes with you, but it still helps. The most practical advice I've had is to focus on getting myself okay, and to switch off worrying about him wherever you can. Right now, you need to look after you. He's an adult, he can find his own support mechanism.
Do not be afraid of seeing your GP and explain what's happened. A lot of women benefit from anti-depressants just to keep on an even keel, and also sleeping pills just to get through the first weeks. Don't be afraid to take a short period of time off sick if you need (and are able to) in order to spend time with your son, and deal with the first few distressing weeks.
I did see a counsellor initially but it didn't really work out for me. I'm now seeing a life coach that is helping me set targets so that I can feel that I have achieved something as I try to sell up my life as I knew it ready to move on after the divorce is over. I take things one day at a time. I come here and rant and rave rather than rant and rave at him.
The best advice I've got from Mumsnet is the 'No Contact' Rule. Clearly, you will have to make practical arrangements for his son, but no emailing emotions, no reacting to any he sends. As neutral and unemotionally involved as you can be.
As you not married, you won't need to worry about a divorce, but you will need to agree access and maintenance. You don't need to do it today, but it's something to think about longer term.
I also recommend a book called 'runaway husbands' which has a wealth of ideas for coping with the first few weeks.
I'm very sorry that you're in this position.