I do not have any experience with this, nor do I have any medical background,but Like the others said, if you feel something is wrong go with your instincts.
Take him to the Dr. and DEMAND answers--maybe ask for a referral to St. Judes!
When my sister was 6, she too was VERY ill, and no Dr. knew what was wrong. After months of my parents watching her wither away--they got MAD as HELL, and demanded answers, that is when they were sent to this one DR. who bless his heart finally gave them answers. She had 3 different diseases--Mumps,Menigitis, and Pancriatitus!!! She finally got better--but ended up missing 6 months of school!
So, FIGHT Mom and Dad FIGHT! I will pray you find answers!