My son is 14 and continues to deal with fatigue, headaches and digestive system problems after his pediatrician of 14 years had missed a severe blockage in his intestine causing malnutrition, anemia, weakness and fatique, What else could be going on?

Fatigue /digestion. May be food intolerance. Would f/u with allergist for skin testing or blood test IgG IgE. Common culprits are milk products and gluten so would try elimination 2 weeks without milk products including yogurts cheese and if no change 2 weeks without bread pasta, ckes cookies. Check B12 I find when low (
Still vague. You've given too little information for us to do much with, and this is more than any of us can handle in 400 characters. Given that there's a lot of gluten enteropathy out there, and a fair amount of Crohn's, and plenty of other things, it's time for a gastroenterologist and maybe a hematologist to get on board.
Not sure of Dx. First of all I need to know what exactly did he have & what type of surgery was done ? What was his clinical condition before surgery? What type of diet he is on ? Does he have any enzyme deficiency ? Digestive system problem is very non specific term which needs to be specified. He might need to be re evaluated by a Pediatrician asap for his further care as he is a growing teenager.