tbh i know people whose parents wanted them to be a doctor but they chose eng/comm, and i know people whose parents wanted them to be a doctor and they did it
usually, people arent happy with their choices if they dont like it and are forced into it, but everyone i know whose become a doctor, even though they didnt want, end up liking it, its one of those things you can really talk yourself into, helping people, good money, reputation, etc.
the people i know who ended up doing comm usually after graduation got a job, realised it was boring as fuq, were getting average pay, and wish they did med, some go back for MD's but most leave it too late and get stuck in a rut,
my suggestion? tell your grandpa you want to go to uom for more prestige and better employment prospects, but tell him how they only have post grad med, do commerce undergrad, fulfill premed requirements and make up your mind after your undergrad degree and see if actuarial studies is right for you