Then we can calculate your very own CPI and see where your inflation really is.
That still wouldn't give his inflation rate, just the price increases in stuff he buys. Inflation is not price increases (which can happen for all sorts of reasons), it's a decrease in the value of money.
It's maybe not intuitively obvious, but the simple version is that "money" really isn't a thing in itself, it's a symbol that we can use to stand for any sort of good or service: a bushel of wheat, a pair of shoes, some software development, whatever. Using the symbols facilitates exchange, so that if you're a farmer who needs a pair of new shoes, you don't have to hunt for a shoemaker who needs a few bushels of wheat, you can sell the wheat to the miller for a handful of symbols, then give some of them to the shoemaker - who in turn can give some to the baker in exchange for bread.
Now it seems pretty obvious that for this all to work, there must be a 1:1 ratio between symbols and goods. Otherwise you could just fake up some symbols to give to the shoemaker. Further, this faking increases the supply of symbols, so that instead of needing 100 symbols to buy shoes, you might need 101 or 102. But also, new goods are created all the time (in an expanding economy), so the symbol supply needs to grow to match.
So we've put governments in charge of making the symbols, and making new ones to match growth. But the government has debts of its own, so it becomes awful tempting to pay off those debts with new symbols that don't correspond to goods. When a government does this, it dilutes the value of all symbols (money), and so you get inflation.
Inflation makes all prices rise by an equal amount. Your new shoes might now cost 200 symbols instead of 100, but you can sell your wheat for 2 symbols/bushel instead of 1.