Has the idea that he is allergic to anything they might have at mealtime.. He is allergic to tree nuts which the staff is fully aware of. No matter what I say to convince him that the food is free of nuts he just won't eat. The staff is also trying ....but..Any suggestions?
Also, don't stand over him. Just place the food down and walk away. Sometimes insisting on the patient doing something works the opposite way. Let him eat when he's ready. Or serve him a can of Ensure.
You might also have him evaluated for swallowing problems. With dementia, eating is eventually going to become an issue. It may be that you have to do it sooner than you anticipated. I would read up on your options.
Ours was not resolved well, we had to change her to a new home because we simply could not convince her the food was ok. Hope you are able to resolve this!