I went through this a few years ago when my Mum passed away. My eldest daughter, lived abroad and could not physically be present because of covid. And also, because of covid, she hadn't been able to come over and visit her for a while so hadn't seen her in person.
Trying to deal with my own grief and support her through hers when she was not nearby was difficult. But it was hard for us both. Hard for me, as her Mum, not to be able to support her with her grief over losing her beloved Nanny, but also for her not being able to support me as she would have liked.
I agree with a PP who says, just keep doing what you are doing. Keep messaging, keep Facetiming and keep the lines of communication open. Just keep going as you are.
Can he write to her? Is there time for him to do that and get it sent to her? He may find it easier to put things in writing than saying things to her face to face on a facetime call. At least this way he gets to say the things he wants to and know his words have been heard.
Other than that, just being there for him will be such a great support in itself. If he is struggling further with his thoughts and feelings then maybe encourage him to seek bereavement counselling. It helped me immensely when I was coming to terms with my Mums death. It will help him sort through his jumbled thoughts, doubts, grief and guilt at not seeing her as much as he should have.
I feel for you. Its hard seeing your DC struggling and physically not be able to be there in person to give them a hug and help them.