I'm going to start with the "DO NOT FEEL GUILTY" thing too

You were in pain and did what you had to do. I was not in pain or anything of the sort after ds and dd were born, and I still did accidental parenting and I am currently TRYING to get 32 month old ds and 11 month old dd to sleep in their own rooms. So you are doing way better than I am

I would recommend a few things:
1. Can you fix up his room, like say decorate it with his favorite cartoon character or something like that to make it more appealing to him? If you make a big deal out him having his own room and fill it with things he loves, he could start wanting to spend more time in it and maybe even sleep in it down the road.
2. Does he sleep during the day? When my ds sleeps during the day it takes FOREVER to get him to go to sleep at night. I mean, he can sleep half an hour in the afternoon and that is all it takes to turn him into psycho baby until 2 or 3 in the morning. When he doesn't sleep during the day he's more tired and so he usually doesn't resist bedtime as much. On the other side, we have to deal with a lot more tantrums at night because he's so tired, so we're still trying to figure that one out.
3. Make sure you don't go from one bad habit to another one. Ds used to sleep in our bed, so then I started laying down with him in his bed until he fell asleep. I got him into his own room, but then it got to where I would spend 2 or 3 hours in his room waiting for him to fall asleep. It was torture!!!! My point is, he needs to understand that the ultimate goal is for him to go to sleep in his own room on his own, even if the road to that is a long one.
4. Consistency is the MOST IMPORTANT THING! And it's soooo hard! Some days I just want to say the heck with it and give in, but you can't let yourself do that. And he'll probably cry and won't be happy about the change, but it's something that has to be done and the sooner the better. And it really takes twice the time to form a good habit than it takes to form a bad one. I've found that if I let my ds sleep with me once, it undoes the work of weeks! I have been at this sleep training for the past month, and still they are not getting it. They are getting better, but there are days that when they finally fall asleep I just go to my room and cry because I'm sooooo tired. You have to tell yourself it's worth it though. If you're convinced this is for the best, you will be able to keep calm and kids sense that, so your ds will be able to relax if he can see that everything is ok because mommy is ok.
And I can totally relate to what you are describing with your marriage. Dh and I had only been married 8 months when I got pregnant. We were thrilled, but after ds was born we went through one of the hardest times in our marriage. Lack of sleep and stress and my guilt and anger at him for not feeling the same way I felt, and him feeling like I had turned into some mommy monster overnight was not a good combination. But we got through it and even did it again! You can get your relationship back, but you have to get ds in his own room. You guys are busy all the time, and then you get home and you have absolutely no time alone together at all, that's got to put a strain on your marriage. With our new routine the kids are in bed by 9 at the latest, and after that dh and I get to spend a little time together, watch tv, read, whatever, but it's soooo nice! and then we get to go to sleep together and talk for a little while before falling asleep (even if we do end up waking up with two little extra guests in our bed every morning...).
I recommend going trough the PPD forum too. They are very helpful there, and I learned that PPD can show up pretty much at any time, and the fact that you think maybe you are depressed means you need some sort of support, even if it's only a few virtual shoulders to lean on.
Good for you for not doing CIO again. I personally found it just made it worse. I think it's been so hard to get ds to go to sleep on his own because we tried CIO a couple of times and it just scared him so much, and made him associate bedtime with stress and mommy crying and just all around bad things. I thought it was really sweet that your dh was the one who couldn't stand to hear your ds cry.
Good luck, you can do this, you just have to convince yourself that it's worth it. When ds was little someone on this site told me to bite the bullet and get serious about teaching him good sleeping habits or I would end up trapped in a toddlers room for hours, resenting him for keeping me in there. I didn't listen, and I ended up exactly like that. So this time I'm being consistent even if it's hard.
HTH
Jennifer