I am so sorry he is so far from home. What you describe is very normal. The ED makes them worry about food all the time- obsessive scary, thoughts, about food. As for him "buying in" about the treatment it won't happen. He does not need to believe it for it to work. None of them,have ever said, that I have seen on the forum, "oh yes this will work. I will do everything" the push back is part of this too.
The point is, however, is that he eats to gain weight. The food is the medicine. There are many ways to "get there", some needed NG (nasogastric feedings), then the child was exposed to oral foods and then that escalated to full meals
In another thread you note the exercise compulsion and that he has had a couple of admissions as well as this recent one. With weight going up, and establishing a good food routine, the brain gets better, but it takes months after WR and maybe more weight than the professionals ever thought too.
Are there distractions available to him at the hospital? Ie games, school, (My D did "hospital" school for 3.5 weeks), books etc. Can he have visitors to distract him, like friends, family? Is the hospital a specialist ED unit? You did mention previously he was at a tertiary care centre, so presuming it is an ED unit.
I know it is hard, and harder when you can't be near him. I do hope they get the weight on him quickly. Is there something you can do for yourself while he is in hospital? Some have made many casseroles (high calories) and put them in the freezer for when he gets home. But also, and very important, can you do something good for you?
All the best,