My son is 5 years old and sinse 4 days back his got this heavy fever, his glands are swollen and painful and has a full body rash what should i do?

Have him seen: You did not provide information on the amount of fever or describe the type of rash in any detail. That limits the response. This could be anything from a simple self healing virus with rash or Strep throat with rash that could turn into rheumatic fever if not treated. Have him seen. In the future, the more detail you provide in a post, the more specific we can respond.