Fever again and again. Again and again?

My boy is having fever again and again..before that he has severe throat ulcers( every year he gets severe throat ulcers ,in 3years he got 3 times throat ulcers on tonsils area ) and fever was 103 sometimes he coughs while laying down ..he has some digestive problems like constipation chest burning vomiting diarrhoea...Dr given Calpol 250mg , Augmentin 500 , Gelusil and Vizylac Syrup ...he was 10 days on this medication... After that he got better... medications off...but now he got fever again ... Everytime I visit doctor they gave same medications every time...stool test cbc done all is fine in report...why this is happening again and again? Fever coming back...he has lose weight by 2-3kgs till now..he became very weak... Only eating medications from months..please help