I am going to address one thing here that you seem very concerned about that didn't seem to have been answered as of yet.
You son's rapid throbbing heartbeat that he and others can see and he probably feels. It is called tacchycardia. It is nothing to worry about other than he has a fast pulse. I suffer from this, and have AS and Hodgkin's Lymphoma. I underwent a nuclear radiogram of my heart, called an RVG recently, and although my pulse is somewhere in the high 110's or even 120's at times, the radiogram showed my doctors that I have a very, very strong heart.
One other thing that can bring about a rapid heartbeat, but I am not sure this is so much true with AS patients but I do know is common in cancer patients, is sometimes Anemia can cause a sudden change in blood pressure. This can bring about "night sweats", and this is followed by the heart pumping harder and faster to move the blood and stopping the sweats and chills. Regular blood counts will be able to tell you and your son if he is anemic.