Don't eat nor drink anything for quite some time.
Just don't eat nor drink for extremely long time, till he starts whining about how extremely hungry he is, that his stomach is growling(I think he said something like that), he really wants to drink, just wait till such level when hunger is red, thirst is dark orange, then he'll mention that he feels sick and sign in inventory near your character will pop in yellow and in time will get darker orange.
Fact is...don't eat nor drink anything for very long time.