What Causes Chronic Vomiting Along With Appetite Loss And Fatigue?

Hi! My son is 5. He had two months off school earlier this year after extreme vomiting, weight loss and fatigue. We lived at our local GPs, he did urine and bloods. Only thing that came up was he had an infection somewhere. We were referred to a paediatric doctor at our local hospital who could not find anything to explain this occurring. An energetic independent loud monkey had turned gaunt and had to be lifted in and out of the bath. We were told it was diet and have been on a dairy free diet. When he was sick I used ketone urine strips which went almost black Doctor assured me it is not diabetes. He has been fine for 2 months but if we are back to square one. Vomiting, extremely tired, no appetite. Ketones test black again. When he is healthy and active his tests are normal. Unsure what to do Worried mum, Dallas