Suggest Treatment For High Fever, Redness In Eye And Vomiting

Hi Doctor, My baby is 10 years old and has been not doing well from last 2 weeks. Initially he got high fever with redness in eye and vomiting. We got all the tests(dengue, chickenguniya, HB, LFT, etc.) done recommended by the doctors. The reports were negative. After 3-4 days treatment, he usual get fever once in 2-3 days, but is suffering from severe lags pain(upper part). He even fall down after walking 10-15 steps. Doctors says, its due to weakness and will get cure with 2-3 week time, but i m worried with his condition. He is a normal growing child, please suggest something.