Go to the Thai Embassy in Canada website, and read the visa section, then when you know your options you can decide how long time to stay in Thailand, according to the requirements you can and will fullfill.
By expecting to get a extension, is predicting the future, which may not happend.
So traveling visa excempt, means you have to leave Thailand before 60 days, and be able to prove that with a ticket out of Thailand within the 60 days.
But he has not got the extension yet, only got granted 60 days, and have to show intention to follow the visarules, which mean a ticket out of Thailand within the 60 days.
Or enter on a visascheme that allows to stay same number of days as the flightticket.