This is good that your father is not having any symptoms and most of the times Essential hypertension
The BP reading 170/100 mm of Hg, is stage II and definitely need to be reduced it is scientifically proved fact that it reduces morbidity and mortality.
One should not take granted as he is not having any symptoms so he has no disease many times silent ischemia,and diabetes are detected when they are investigated so it is prudent to get his medical work up including ECG,FBS,PPBS,Creatinine,Lipid Profile
,Uric acid,Vitamin D3 level,and a 2 D Echocardiography
and if required stress test or CT angiography
Looking to his age Calcium channel blocker
is preferred drug but depending on his response he may need to increase the dose or add some other drug.
When such drug is started cerebral circulation is affected hence he feels weak and sleepy but this all will hopefully passes in few days.