Sometimes when they get old their teeth are so smooth from having them done that they find it hard to eat course long fibres. He may be able to eat chopped hay as the fibres are shorter, or possibly carrots.
Fixing this in an old horse can be a major job. They may need to use morphine to anaesthetise the horse during the operation. There are different ways of going about it, but there is a risk because they are using powertools of damaging the pulp which could be extremely painful. Afterwards...