Simply focusing on helping your child eat is not good enough: the mere fact that he has been having fever, especially that it is high-grade fever (above 38.5C) for 4 days now should prompt you to have him seen by his pediatrician today, to rule out a source of infection, like the tonsils, ears, lungs or urinary tract (less likely). He may need antibiotics as this does sound like a serious infection.
Moreover, since he has been vomiting frequently and not replacing this loss properly (you have mentioned that he was not eating well; I do hope he is getting enough fluids though), he may very well be very dehydrated. Now dehydration in and by itself causes a rise in the level of "acid" (or ketone bodies) in the blood, which renders the affected person nauseated all the time and curbs his/her appetite. Adequate rehydration is thus vital, and of your child is not able to hold food and fluids down, he may require intravenous (IV) hydration. In order to determine whether or not IV fluids are needed, your child must be examined by his doctor.
Meanwhile, make sure you provide him with oral rehydration solution around the clock. Again, visiting the doctor's office is a must in his case.