He is in his cave.
He is being man the protector. He believes he has to do this job to provide for and protect his family.
When men get into cave mode it is very hard to get them to think properly. Merely having a conversation with him will not be enough - men do not react in the same way to conversation as women do. You will be having a conversation with him about the stress, the job, etc, and how you do not want him to become ill but it will NOT be going into his head.
You think it will be but it will not.
This is what you need to do - seriously, this is how you get a man to listen when he is in cave mode.
You need to find a quiet moment - create it. No kids, no cooking, no TV on in the background. Nothing except peace and quiet.
Sit him down in a chair and sit in a chair directly in front of him. Take hold of his hands. Look him directly in the eye and then, and only then, tell him in simple, plain English of your concerns and worries.
You need to tell him that you are worried about his health. You are worried that if he becomes ill from stress that you will be worse of than you are now.
You need to tell him that you need him to go and see his GP about stress and to get signed off sick from work due to stress.
You need to tell him that he needs to do this for his own health and for the safety of your family. You can point out that it will also give you, as a couple, time to think about how you will survive financially if he has to pack his job in.
Tell him anything else you feel you should say but do not over-whelm him with things as this will make him miss the point and dismiss things - the less you say the better as it will get into his head.
Men only truly realise that something is serious when women talk to them in this way - quiet place, no distractions, sit in front of him, hold his hands and look him in the eye.
Best of luck.