Teach him to put his money where his mouth is. Agree on a bet, then tell him to ask the question to his maths teacher and come back with the result. Although a bit odd that he doesn't know how to convert fractions and decimals at his age. My daughter's learning this now but she's 9 - and I teach my 15 year olds more complicated stuff like Pythagorean Theorem. :/ <shrugs>

Anyway, losing a wager might teach him a lesson. I sometimes have kids in my class who engage in stupid arguments like this and I tell them to make a small wager (e.g. loser buys the winner a pack of chips from the school canteen). 9 times out of 10 the kid who's wrong will very suddenly back down.