What Causes Weight Gain, Recurrent Diarrhoea, And Vomiting?

hi, my son is 3 yrs old. he has a history of fibrile fits. once a yr. happens only once in an illness. and lasts less than a min. last fibrile fit was with phyrangitits 2 months ago and was given iv.he has always been a slightly healthy child since birth but gaining excess weight since he was 2 yrs of age. at the moment his weight is 36 kg for the past 1 wk he has developed bouts of nausea vomiting and diarrhea. happens once or twice a day. settles for one day. he refuses any food. takes sweet food items only. refusing to take any medicine. i have started probiotics since yesterday mixed in his food. kindly advise? and how to manage