Quantity of Food for the Ill

For someone who had mononucleosis and was instructed not to fast, how should this be done? Should there be a certain limitation as to how he eats (to start fasting, to eat only as much as needed, measurements) or can he eat normally?

Shalom!
Most people who are ill can eat and drink plentifully close to the fast, and from then on not eat until morning. [Except for diabetics and the like, and anyone who received different instructions from a doctor or rabbi].
There is no need to eat less, but one should not eat for pleasure [sweets, ice creams, etc.]. One should eat what is necessary for the body to keep strong.

Shulchan Aruch, Orach Chaim,