Some days, the stories here just break my already broken heart. Please accept my hugs and know how much I hurt for you. My bff has been a rock through all my SAWH's issues with his AP, his prostitute, his escorts, his hook-up sites, his EAs, his newfound atheism . . . If she'd been taken away from me along with him, I'd don't know where to turn. But that's where you are. And you need advice. So here goes.
Generally, along with making no major decisions for 6-12 months after the A, I'd also tell few people. That's not to say I think your WH's sins should be secret; I simply think you need to be in a better place, rationally, before you act in an irreversible manner. You can choose to not follow through with divorce proceedings once you've initiated; you can't take back telling people once you've told them. That goes for your children.
You need to be honest with your son. He'll be watching you now and you won't be able to fool him. It'd have been hard before but will be next-to-impossible now. Give him the bare minimum. Tell him you'll share more, in an age-appropriate manner, once you have a handle on things yourself. He needs to know that one of his parents is incapable of lying to him. He needs to know that your changing the story (lying about his dad NOT having an affair) was a one-time aberration because you were in such pain viewing his. He needs to know that while everything may not work out painlessly, they WILL work out and you (and your WH) DO love him. He needs to understand that there will be some changes at home (your WH moving out of the house or the master bedroom), your break with your former best friend, your moodiness, your increased time on the computer (researching and coming here), etc.
Another bit of advice? Find someone you CAN share with: a sister, an ecclesiastical leader, a therapist, a co-worker/friend. Bottling this up may kill (exaggeration) you. I kept it inside for a month and thought I'd lose my mind pretending to the world that all was well. A journal (password protected) may really help you, too. Mine has A tidbits, my vents, my rages, my tears, proof, photos (of the prostitute and of the OW), etc. and I don't have to worry about my kids or WH finding it.
Also, please take care of yourself. Go to the doctor for STD testing--the whole gamut. Eat, drink, breathe, sleep.
Schedule nice things for yourself: mani/pedis, massages, a shopping spree, a new haircut, a trip with or without your kids, a good book, bubble baths--whatever you consider an indulgence. It's time for that because you have to REconvince yourself that you have worth. Intellectually, you may know that, but emotionally, you've been dealt a shocking blow.
Be gentle to yourself. Don't beat yourself up for missing red flags or for not seeing them at all. You trusted because you are trustworthy. That's actually a very healthy trait. Don't believe any of his potential blameshifting. Don't believe popular culture's finger-pointing, either. You are pretty/skinny/smart/loving enough to be cherished and protected by your mate. Whatever your imperfections, you are worth fidelity. Every spouse is. In every marriage. It's NOT your fault. None of it. Not one, single, broken, cheating thought in your WH's mind is attributable to you. You are amazing!