Too many unanswered questions here, to get any further than the excellent replies from above.
What was the original illness?
What investigations have been done?
Which Specialists have been consulted?
Is his problem going to improve, remain the same or deteriorate?
He drinks well but eats slowly means that his nutrition is at the moment, not a major problem.
If it does get worse, then he may need an indwelling feeding tube, but at the moment, I would agree with your son to "monitor the situation."
I have a few diagnosis in mind, but at this stage they would be pure conjecture and not worth serious consideration.
Interesting situation....keep us informed.