Does he have a favourite food he might eat. Maybe see if theres anything he really wants to eat - ice cream ( easy if throat sore) or jelly perhaps.
My two went through a phase of leaving there tea, sayiny they were full, or just taking forever, so we stopped all computer/ console play and started a reward chart.
Would somethinh similar work a star if he eats afew spoons, 2 stars for 0 spoons etc. When he gets x stars a reward/ treat ?
Hope he gets better soon.

its such a worry when they won't eat.