Hi, My 3 Yr Old Son Has A Very Croaky

Hi, my 3 yr old son has a very croaky voice and has a runny nose and phlem and is weazing when he sleeps with mouth open, he has lost his apetite and eats hardly anything all day but drinks liquids. I have taken him in to see the doctor but all he has prescribed is vitamins. Is that enough. I am worried that it could b emore serious.