Talk to Pediatrician on Stomach Problem

My child is almost 4 years old. He is very weak. He doesn't like to eat food. It's quite a tough job for me to make him eat. I try to offer him his favourite foods but stil he eats very less and takes a lot of time in chewing every bite. And after eating he goes to washroom to pass stool, like 3 to 4 times a day. I don't know how to treat this problem. He otherwise is active but gets ill (flu, sore throat, fever etc)every other month. Today he is vomiting and has some sore throat and light flu also.