What Causes Vomiting, Low Appetite, Fever And Weak Legs In A Child?

My 9 year old son had vomiting for one day, a fever for two days prior, and now is complaining of weak/sore legs (2 days after vomiting incident). We are now on day 6 of feeling bad. He is not eating either, says that he feels full if he has one bite of any solid food. We are worried he isn't eating enough and that he has this weakness in his legs. What may be causing this?