I don't want to focus on his math mental skills but his overall education was better. He can also beat people in language/words associations.
Well, _different_, possibly. Neither word associations nor mental arithmetic are the be-all-and-end-all; mental arithmetic in particular has been more a party trick than anything else since the advent of the pocket calculator.
What's hard is not _modifying_ your IQ, but increasing it.
Is mental arithmetic practice not part of a general math education curriculum?
You'll be able to improve your mental arithmetic performance by orders of magnitude from pure memorization + practice alone, but this is not how people typically learn math in general, and rote memorization will not make you a good mathematician in the long run (it's probably not even good enough to get you past university-level exams).
I too am a maths graduate.
My peak mental arithmetic was about age 11 by then I had learned times tables etc by rote and could deal with money - being the UK this was LSD so had to convert between 20 and 12s as well as decimal. Then in the next few years we want decimal so no need for that daily task of converting and we got calculators if we needed figures so little practice - I could still keep track of what the total amount I would pay in a supermarket and calculate change quicker than the assistant using the till but even that has gone as you just pay by card.
Nowadays do you need to be good at mental arithmetic, probably for doing quick estimates but I don't know if educators think that getting mental arithmetic working well is needed.
You don't need a 'better general education in math' to be good at mental arithmetic. In fact general education in math' is pretty much irrelevant to mental arithmetic.
To be better at mental arithmetic, you need to learn and practice mental arithmetic.
I am willing to bet your father did exactly that when he was younger, compared to today's 20 year olds?
Your father might or might not also have had a great general math education. But that's pretty much irrelevant for the topic at hand.
(I'm a mathematician by training, so I have a pretty good 'general education in math'. But how good my mental arithmetic is depends very heavily on whether I've recently had reason to practice.
I was my best at mental arithmetic in about 7th and 8th grade. 7th grade is when calculators are first allowed in schools where and when I grew up. If, like me, you keep forgetting your calculator at home, that means all of a sudden your mental arithmetic gets a lot more of a workout than before.)