Anna I hope you are able to do what is right for you all. I noticed from your post that you said your partner said he was ...unhappy and thought probably best if we split up. That is not exactly the same as 'I must leave now!' It sounds as if he has recognised he is unhappy and is looking for ways to change that. Of course you cannot force him to stay or force him to change but could you both move into a new phase in the relationship?
I could be way off base here but I wonder if it is your house and he lives there, maybe his job is not so great and he feels like less of a man, apologises I am a feminist but I am just wondering how he may feel!
You both argue and have been together a while and maybe he is hitting that stage in life/ in the relationship where things are not going well and is he thinking what next? Is this it? Are you thinking that, or would you be if you were physically more healthy? That for either of you might be leaving or it might be a change in the family dynamics.
Of course you have to think of your son but I am not sure your posts have implied he would end up as your carer! Or have I missed something?
If you are the best person to parent your son at age 5 I certainly would not be sending him off to live with his dad miles away (if I were in your shoes). I wonder how many of us would want to do that. Your partner wants to leave, I would not want my son too unless I really could not care for him.
But the suggestion that you all move nearer his family for the support, might that work?
Another option if space allows might be a trial separation where he stays in the house but you live separately, or at least sleep separately while you work on your issues. I am not sure I would recommend this but just wondered.
I do think you are portraying that your need of each other is very practical and that may well not be the full story. Many couples are quite different. My dh is very different to me, he is very sporty and I am not! We hav ebeen married almost 14 years and are still pretty much chalk and cheese although we have moved a bit closer to each other personality wise! Yet still I am very sociable, and he is not. But we love each other and have found things to do together that celebrate that, while also be very different people. We have also argued a fair bit in the past, but have both worked through issues, like fertility issues, and come out the other side stronger.
Ultimately, if you do manage to do this I think you must be able to see each other as man and woman, loveable, desirable, etc etc and not just someone to put a roof over the head or someone to support you when not so well. You need to find the passion and love, either again, or for the first time. Whether you do it or not will be up to both of you. But I would not write him off, he doesn't sound totally out of it yet, and I would do all I could to make sure he did not write me off, if I thought we really had a shot at making it work. No one else can live your life for you. Only you know what is really going on in your heart. I think to write it all off without some sort of fight would be unethical.
All the best. 