If you are wearing a suit, a smartphone makes the trouser pockets bulge a lot, so it's common to keep it in your shirt pocket or jacket inside pocket - Both of which is next to your heart.
1: https://www.fda.gov/radiation-emitting-products/cell-phones/... 2: https://www.heart.org/en/health-topics/arrhythmia/prevention...
Same with smoking, drinking, taking drugs, etc.
He doesn't have a pacemaker, but it may not be as unreasonable of a thing as you might assume.
ex: https://oldnavy.gap.com/browse/product.do?pid=3397220020504&...
It's the most convenient place for a phone on your person.
Easier than pants pockets if you're sitting down. No fumbling with inside jacket pockets. And you can use your "normal" jacket pockets to keep your hands warm without worrying about your phone falling out.
