My 3 year old vomiting milk and doesnt want to eat.

My son is a very picky eater and his diet is limited to few food items only and I always have to feed him. Since two weeks now he has started throwing milk after his mid morning feed. At this time he is at school and later he wouldnt eat lunch aswell. When he comes home at 1 pm I feed him lunch and he eats it (not very happily though) and then eats something in the evening and later drinks milk. He doesnt want to drink milk or water and eat food. He is very cranky, screams at me and gets very irritated. I have been to doctors they say its viral and he has a bad throat that why he doesnt want to eat and is throwing up. I need some guidance from you if you think there is something else that I could do to help my poor boy.
Thanks a lot!
Zehra
Singapore