please excuse the 1 handed typing. am eating lunch with the other.
you do have a problem. can you put up a copy of an invoice on the wall of wherever the feed is kept. he may be shocked by it esp if belonging to the old school.
the trough must be clean and dry. if there is anything left over from the last feed then they are being overfed. clear out the old stuff so it doesn't spoil the new feed put in the trough.
Back to two hands.
We had a long term employee who was, quite frankly, a liability. Luckily he voluntarily moved away and the problem was solved that way.
Can you explain the financial side to your parents? I'm sure they'd hate to think that so much of your money was being wasted. It must be difficult to address the problem, especially as the person is presumably causing the same sort of problem to them. Can they explain the financial side to him and say that they really need to cut costs and have re-calculated the amount of feed needed and it now needs to be 1 small bucket a day instead of 1 medium bucket, or whatever? Can you deliberately keep the stock of food low, just a day's worth at a time, perhaps?
I don't know if any of that helps. I do sympathise, though. I'm not farming now, unfortunately. I moved away myself and do office work now, believe it or not. Farming is still in my blood. Ah, well.