It wouldn't hurt to get an mri of his back, hope things get resolved!!
He only had an EKG and no other Cardiac testing? If the answer is yes, I would recommend consulting a cardiologist.
He has had his inflammatory markers checked? ANA, RH factor, Sed Rate?
Sounds either cardiovascular or autoimmune.
I am very sorry that your son has been struggling with these symptoms. I am not an expert by any means, just a mom. A mom that has a 16 year old daughter with a condition called, Chiari Malformation. Does your son ever have headaches??
There are many, many conditions that could very well be the cause of his symptoms. It definitely sounds like the doctors have failed to really get to the root of the cause. And I pray that he does NOT have Chiari.
*IF* he should have the symptoms of Chiari, I would recommend seeking out Dr. Michael Rosner in North Carolina. Just Google his name, and his website will come up. You can email him with your questions, and see what he says.
Like I said, it's very possible that it is not Chiari, but just in case, do some research on it, as your son't best advocate! Hang in there and don't give up! It took us nearly a year to find what was causing my daughter's slow and painful decline.
(((((HUGS))))))
Blessings to you and your son,
Rebecca