He doesn't have to eat... just keep hydrated so he does not get dehydrated.
Have you ever had food poisoning or stomach flu? It is VERY hard to eat when sick like that. When he is better, his body will naturally be 'hungry' and he will eat.... but just not right now. Its okay.
But, I would really try and get him hydrated... a sip at a time. When sick with a stomach flu, the body actually CANNOT intake a lot of liquids at one time, or it will be thrown up. So... take a little sip at a time... over the course of 1/2 to 1 hour.
A person sick with a stomach flu, simply cannot 'drink' like a normal person either. The body cannot take it.
McDonalds will just make him throw up.
If he does not drink anything, and gets dehydrated.... (look online for the symptoms), then he may have to go to the ER and they will give him an IV to re-hydrate him.
Tell him, is that what he wants???? Dehydration is very serious... more serious than not eating.
So, encourage him to drink something.... or suck on ice too. Popscicles etc.
But you do not have to 'force' him to eat solid food. He has a stomach flu... that will not help.
I would REALLY make sure, he is not dehydrated... his weight loss... can occur from not drinking and dehydration.... not from food.
Think about that... because, a body typically loses water weight first... not muscle weight.
So, you need to keep that in mind....
WATCH out for dehydration in him... especially since he has NOT even drank anything much... since Wed.
All the best,
Susan