I hesitate to suggest anything in case it puts you in a tailspin. I'm nothing remotely like a health care professional, so please take what I say with some caution: it might be a tumor on his adrenal gland, or he has an endocrine problem with his hypothalamus or pituitary gland...not sure which or if. Unless he has an electrically disordered heart, and that can be determined by a mapping procedure (catheter through his groin up into the heart and a probe looks for spurious voltages where they shouldn't be), he is probably responding to adrenalin rushes, perhaps due to vivid dreams, or for some other reason.

An electrophysiologist is a heart rhythm expert who can map out his electrical system for malfunction. They can 'challenge' the heart using several drugs, from caffeine to epinephrine, from isoproterenol to adenosine, and make his heart beat quickly and become stressed. They'll find the place where the disorder lies. But if they insist that he's okay electrically, then there is an endocrine disorder or a tumor...that's all I can think of.

Keep pressing the various experts for a diagnostic that might show something. And, I wish you hurried success.