It could be, especially if he's skirting around the edge of heart failure and other problems associated with metabolic disorder and hypertension. Healthy kidneys do more than a decent job of keeping electrolytes of all kinds in balance, even when one overdoes it, but when endocrine and metabolic problems are in the mix, kidneys don't need the extra effort it takes to keep things regulated. In case you aren't aware of it, the kidneys are a strong monitoring system to keep your electrolytes in balance, and they help to keep your blood pressure in the 'normal' range.

I would, with your physician's agreement and monitoring, suggest that you start by mixing a quarter tsp of salt in distilled water and drink two or three cups of it a day (you can buy a gallon of distilled water for just a couple of dollars, or so, at large stores). Take your morning BP, ideally before tea or coffee, and keep a record, a chart, to see if there is any spike or rising trend. If not, then your current regimen is working, and you'd be able to drink softened water.
https://www.ahajournals.org/doi/full/10.1161/JAHA.119.012758