Doc doesn't want to prescribe beta blocker

[DesertRain]

My 31 year old son has developed tachycardia after getting his 2d shot of Moderna. He has been to a cardiologist who gave him an echocardiogram (no stress test) and then sent him on his way. So he finally got a primary care that started him on propranolol which is working wonders (tho he needs an extra 25mg after strenuous exercise)...now after a couple of months on the propranalol, doc wants to take him off, start him on Lexapro, because he's 'too young to be on a beta blocker'. I myself have been on atenolol since my mid 30s so I am not getting why the doc is taking him off something that is working...