Sounds like PVC's to me!
Cause?
People with high levels of dopamine and take hi levels of V-C causes higher then normal voltage spikes to the heart and the additional V-C raises dopamine levels even more. This is rare!
Most common in this type of situation are people who take MAO inhibitors and high levels V-C see a lot more of this then above. If he takes MAOI's reducing the V-C and taking it with more time from his meds might help.
Or Owen could be right about the stevia. Need more input to give a good guess