My son was diagnosed with stage 4 cancer in 2011. He completed chemo, radiation, stem cell transplant the same year. All scans, have been clear with last scan being November 2013.  He started vomiting last Saturday in the middle of the night and has been vomiting once a day only in the middle of the night or around 5 am.  He does not feel bad prior too or even after.  He also had  diarrhea  as well. On Wednesday he came home from school completely exhausted and only wanted to sleep.  Thursday he complained about his legs hurting really bad that he could barely walk or tie his shoes.  His appetite has also decreased where he only eats once a day.
HIT hank for asking to ChatDoctorI really appreciate your concern, your so is having malignancy and taking treatment for this now he lost is his appetite this could be because of the disease itself and the side effects of therapy and second possibility of some gastrointestinal infection these all needs to be examined, but for the symptomatic relief he can be given "Tab Ondansetron" for the exact cause he needs to be examined clinically, hope this information helps you have good day.