Welcome AnnaR. My heart breaks for you and your son.
While it is different when it is your own family, I'm confident that your experience from hospice will serve you well. Distance of course complicates things as body language gives us so many cues and sometimes touching or just being there is what is momentarily needed the most. But I'm only telling you what you already know.
The fact that you understand anticipatory grief is helping him. So many people find it difficult to be frank about losing someone before they're gone. It sounds like you are giving him that safe place to talk, vent, cry, and letting him know that all his emotions are valid and okay.
I'm going to reach out to some other members who can come share with you. In the meantime, here are an article and another discussion thread that you may wish to read.
Do your son and daughter-in-law have children? What family supports do they have close by (geographically)?
Colleen