I would suggest going to and endo since his thyroid is involved thyroiad can cause many symptoms
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I am sorry to hear about your son. Enlarged spleen is seen in leukemia, malaria, tuberculosis, lymphomas, hepatitis, cirrhosis etc. Since he is losing weight and vomiting—his liver and pancreatic function should be assessed. He could have caught hepatitis.
Also consult a gastroenterologist to rule out severe acidity, acid reflux, IBS or Crohn's. Other causes I mentioned should also be investigated.
Since I cannot examine your son and know other related conditions he may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP for primary examination followed by proper referral.