Son is overseas and is struggling with mental health and Marriage

Just tell him you will be there for him if/ when he needs you.
It sounds like he is at a crossroads , but he will get through it, best wishes , it is hard when it is out of our control .

I don't know what I can say that may be of help bevruss but didn't want to ignore your post.

Our DS lives in Aus. and although his marriage ended by mutual consent it was still a very worrying and upsetting time, made worse because he was so far away and we couldn't see him.

There's little if anything you can do while he's in denial apart from offer your support and gently encourage him to accept what is actually going on.

You're ongoing understanding and support for your d.i.l. will I'm sure be a great source of comfort to her, and maybe another way of supporting your son.

I hope that a resolution to your son's problems can be found
.

He refuses to take meds or go to therapy. Yes, I've been there and it is incredibly frustrating. Maybe no comfort but we are all apart at the moment so in some ways it doesn't matter how far away he is.
He has a job. He has a good wife by the sounds of it. He has a mum who wants to support him.
I would send him short messages for which you don't expect a reply. Tell him you love him (presumably you do!) but don't quiz or put pressure on because he will only respond when he wants and how he wants.
Although it is good that you get on with DIL don't take sides. You live far away and don't have the full picture.
It is hard I know, but just do what you can to take the pressure off yourself and hope that one day it will be sorted.

That must be such a worry for you bevruss it's hard when you are so far away I know.
Let your son know you love him and will support him and your daughter in law as well.
Use technology to keep in touch without stressing him. I had a difficult situation when my daughter was in USA before the days of internet etc. I ran up a very large phone bill supporting her but it helped her to let off steam and have a cry. They have sort this out themselves so try not to stress about things. I hope he gets some help soon.

What everyone has said really covers it, it must be so stressful. Just be there, dont take sides or give advice. Somehow they will work it all out.