Hoo boy! Best place to be at an awful time, pie. It sounds like you and your H are somewhat opposites. Can be fun, but not when there are difficulties in the M.
As stated, it is time for you to GAL, find out who you are. I bet you're not the whimp who allows her H to do and buy anything he wants. I bet you're not the doormat, that allows H to get out of taking care of S4 now and then --- alone. I know he baths him and puts him to bed every night, but does he take the munchken out to the zoo or the park, or any other fun thing. You build R's with your kids by experiencing the outside world.
Time to find your inner self, the one who is in control of herself, has boundaries, and has fun with friends without S4 or explanation to H. If he wants to leave, then so be it .... BUT show him exactly who he is leaving, not the shadow, but the substance. You've allowed him too much stuff, and it doesn't make one happy, ultimately. Being your true self, being there for your family, honourable in business, friendships and family ... I contend that that's what makes one happy. But, he has to find that out for himself.
Here are ways to limit contact .... don't always answer the phone or text, don't answer emails immediately, only contact when it concerns S4 or finances, keep contact businesslike and distant, if it can't be helped and you have to be in the same room as him then look your hottest (casually so without making it look construed), look happy when he's around, don't allow him to EVER put you down when in company ever again (he probably won't now since he has left, but if it happens, you make a point of stopping it in front of the same company). Anyway, that's enough for now. You probably feel very overwhelmed, so don't forget to breath deeply now and then to calm yourself. I found meditation helps a lot.
I am glad you are going to the gym. Exercise is good for the mind and the body. And being distant when he comes to bath/tuck S4 into bed is the way to go.
I agree that you should be prepared for an affair. People seldom leave unless there is someone else to go to, especially men (sorry guys, but it is true I think). I thought my H was too busy to have an A, but what he had was a long distant EA emotional affair); they talked on the phone constantly, emailed, texted and so on. She lived far away and was an old school flame.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim