Prayers sent, for him and you and your wife and the rest of the family also. Just throwing this out there, but have they checked his magnesium level, not just the blood level. (His blood level could be fine, and he could still be deficient in it, as his body will leach it from his bones to keep a certain level up.) And his potassium levels? If it is OK with his doctors, ask them if when he has his next case of tachycardia, (big word for rapid heart beat) if it would be OK to give him some magnesium supplements, and some potassium supplements also. God bless, and I pray that all ends well with their search for the problem.
Wayne
P.S.
If you do not want to take supplements, Bananas are high in magnesium, and potassium also.
Wayne