I would suggest you use a balanced cal/Mag 1:1 supplement such as Integrative Theraputics cal/Mag 1:1 liquid. Also drink coconut water to add the potassium. The potassium supplements can be dangerous but the food is not. The coconut water is higher in potassium than bananas.
This cal/Mag, potassium approach is the only thing that has ever stopped my arrhythmia. lowered my blood pressure. Too much magnesium will give you heart failure. Too much calcium will cause a heart attack. That's why you want to take the 1:1 ratio.
The potassium is because everyone has low potassium and it works with the cal/mag to counteract the effects of too much salt.
Start with the coconut water to see if it helps first. Then raise the amount slowly as you add the cal/mag liquid. Do all this slowly because the heart has to recover while you are doing it. If he feels bad at all STOP.