I remember well the rapid heart beat problems and yes, it is a very worrying symptom. I really thought I would not be living very long smile
However, in getting rid of the AS by avoiding starch and taking a lot of mineral pills, I solved the problem.
I tend to see the palpitations [in fact I think it was worse than that] as mineral deficiencies. Now, do not ask me which one because there are several mineral or vitamin deficiencies that tend to produce heart irregularities.
Mineral deficiency is part of the AS picture because of the damage that AS does to the cartilage and the damage done by inflammatory products. I believe that some of us get into more strife too because we have had mild mineral deficiencies to start off with. It is not something which Doctors seem to be trained in....- to be fair the signs are somewhat subtle.
The other consequence is that if we start to be liberal with the drugs we then find some minerals/vits are depleated even further.
I hope that your son gets to try avoiding starch first to see if it is the AS reaction [he will lose all the pain if so] and to supplement with minerals [a general plus trace mix] to see if the heart troubles go. Neither is very drastic, but simple to do, and the best way to start both diagnosis and treatment.

PreAS = STARCHfree, yoghurt, ,, meat , vitE, niacinamide,B complex
Fibro = antiviral, MAGNESIUM /B6
Arthritis in fingers = no potato fam. plants
Candour & laws differ from place to place