Really sorry you are going through this, OP. It really sucks. My verysoonTBEX H left just over a year ago after 22 years together and 16 years married. My now 15 yo DS1 is just fine. I was completely honest with the DC without slagging off their dad, as they'd had enough lies. Just did the we both still love you, it's not your fault but your Dad doesn't love me anymore, he loves the OW. Ex now lives with the OW and my 3 DSs spend every other w/e with him quite happily. (I'm not happy that she spends so much time with them, but their need to have a relationship with their dad comes first.)
You might find that your DS comes down firmly on your side as teenagers can be very moralistic and will see it in very black and white terms. If so, try hard not to use him as your emotional sounding board, not healthy for either of you. My DS1 appreciated being treated honestly and I did keep him informed about what was going on with arrangements etc. He has grown up a lot over the last year. 
Others will have better advice, I'm sure, but do get a good solicitor and make sure you are compensated for any childcare vs career choices you may have made as a couple. Don't go for 50/50 custody unless it's what you and your DS really want, as my DSs were dead set against it, as was I. I'm sure for some EX Hs it's more to do with reducing maintenence costs than for the benefit of the DC. At 15 your DS's opinions will count.
(((hugs))) You will get through this. A bit of anger and the moral high ground will help in the short term. I lost 22 years worth of comfort eating weight, half in 2 months, the rest over the next year. Every cloud, eh?