2 year old vomits after each meal

my 2 year old has suddenly started to vomit after each meal for the past 6 months now. The doctor gave him Zantac which worked for a while but looks like its not working any more. We went to a feeding specialist who put him on lactulose stool softener as then his poops were very constipated and had blood with them. He started getting better for a week or 2 but since the past week now.....he has not been able to keep any solids inside his tummy. HE throws up chewed food as it is....no liquid with the food at all......its just big blobs of chewed food coming out. It is so bad that tonight when he had food and went to sleep...i was happy that he kept something in there....but right now....he just woke up from his sleep and puked. I am really worried about him. Please someone advise me on what to do.