Thanks for choosing HCM.
You have done the necessary tests which have come back as normal.
It could be stress
Keep his B.P. monitored once in 3-5 days for next 2 months.
Give him salt restricted, low fat diet. Stop the junk foods. Ask him to regularly exercise- even walking 45 minutes will do.
Please get an abd-pelvic ultra sound
scan done to rule out any problem with his renal arteries.