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6 Months.....so what day did he enter Canada in May?

May 17th! Looks like we’ll be alright then! For some reason I was going off 180 days!
 
He has to have valid status in Canada when he does the landing. I, personally, would apply for the extension....today. Rather spend the $100 and be sure he can complete his landing than not and find out he can't land because he's out of status.
30 days is recommended to apply for an extension, but not required. As long as it's received by IRCC before the expiry date, you'll be fine.

How are people who are out of status managing to land as PR? I thought the AIP basically gives you the status back to land as a PR?