I am a SAHM who also studies fulltime, and I work casually - well I haven't worked for a couple of months though so I'm pretty much at home most days - I'm not actively contributing to bringing in money at the moment.
For my DH and I, the first 18 months after DD was born was the hardest. We had a constantly evolving little person that we had to adjust to and also having to adjust to our new roles as parents, I also went back to work fulltime by 12 months. And it was really HARD.
I believe as a sahm that the house is my responsibility and so is the primary care of DD. I'm not saying that it is an easy job by all means, and there was a lot of contention as DH didn't feel appreciated. I had to actually think about where he was coming from to understand and help us move forward.
For my DH, coming home to a reletively clean house with dinner started, helps him to relax. I want my home to be a sanctuary for all the family. By doing this, it helps to show him that I appreciate what he does, he works hard all day so that I can be at home with DD, and I can also go to school and finish my degree.
My house is not the cleanest house. There is washing piled up, toys and stuff everywhere, but I am working towards making it better, and he can see the difference when he gets home.
I think that it is important that your husband spend some quality time with your son too, but maybe he would be more willing if you asked him to do something that he see's as "fun". I guess I can sort of understand that if he works a lot, the last thing he would want to do is change a nappy, yes I know that you have to do it too, but you also get to spend all day with him doing other fun stuff too.
Maybe they could sit and read together, or he could take a bath with him? Or just muck around as boys do.
One of the biggest problems that I found for me was that neither one of us felt that other appreciated the other person. And I had to be the bigger person and take that first step. I had to be the one to make the effort to show DH that I did really appreciate what he did for us as a family. What they do is important. (What we do is equally as important, but sometimes I think that we forget that what they do is just as important. We are a team)
I also had to give DH space when he got home from work. He needed about 20mins to just seperate himself from work. He would sit on the computer, or listen to music or whatever, then he would come out when he was ready and that is when I would talk to him, as I found myself talking "at" him as soon as he walked through the door. My need to have an adult conversation was the driving force
I was ready to leave DH during this time as well. So I do understand where you are coming from. And what helped for me, may not work for you, and the underlying problem for me may not be the same as for you.
But if you want your marriage to succeed then you need to sit down and work out what the underlying issue is.
He probably feels just as overworked and under appreciated as you do. You both work hard, you just need to find where you both fit now, as your roles as parents are different to what they were before you had your son. (I'm assuming he is your first and only child?)
I try to aim to have the lounge and kitchen in some sort of order before DH gets home, and it makes me feel better too, having some area of the house that is tidy. Try to get your son involved, make it a game. Do you have a toy box that he can put his toys in? Have a race to see who can put the toys in first.
Talk with your DH, tell him that it is important for you and your son that he spends time with him, and ask him what he wants to do with him? I had to learn to get out of the way when DD and DH spent time together, having me hover in the background correcting and waiting to swoop in and take over was really not helpful to DH!
Does your son still nap? Could you take some time when he naps to just give yourself a break? It sounds like you are both stressed, and tired.
Can you go outside? What if when you do get the place tidy that you take your son out of the house? For a walk or a play in the garden.
Sorry! I know this is really long, and I don't want to come accross as a "know it all" It is hard. And I never though DH and I would make it, honestly. But we did. And it wasn't one of those things that one heart to heart solved. We had many, many discussions about it. Many, many arguments about it. But eventually we got there.
Chances are that you would get primary custody as I think the tend to like the "status quo" in regards to who has been the primary carer of the child, (But I am not a lawyer!) but do you think that is really what is best for you and your son? (not just your son, but you too!) You fell in love with your husband, and he you. It is still there!
I always tell me hubby that men are easy, a tidy house, dinner on the table and a bit of action every now and then and they will be eating out the palm of your hand.
And I think we all tend to be more giving when we are feeling that our needs are being met.
Anyway I hope that somewhere in there you may be able to find something that is helpful. And if I am totally off base, I'm sorry. I wish I could give you a big hug. No-one really tells you how hard it can be to make the transition. Especially when you are both feeling overworked, underappreciated, and tired.
Please let me know what happens, if you want to, my PM box is always open.