It is better to get pre-cleared before traveling, that way he won’t get hung up at the border. Canadian immigration does not have to let him in at all, so if he shows up at the border and they deny him, he won’t just be delayed, he won’t be allowed in.
And, worse, once denied, it will never be easy to get into Canada. Comedian Mark Maron has talked about how one time he was cleared to fly to Vancouver but decided to drive instead. The Canadian border guards held him up for half a day, And every time since then when he goes to Canada, it takes him forever to get through, because he is flagged in the computer.
The US and Canada signed a treaty sharing criminal background computers after 9–11. Canada didn’t care before, now it flags anyone with even a drunk driving. Get him cleared before he goes. Call a Canadian Consulate or Embassy in the US to ask how.