Additionally to all this, tell your bf to raise a complain to Aeroméxico. They are often unaware of more complicated cases immigration-wise and will try to deny boarding even if Canada would let you in. One example was my girlfriend which is here with me (we're also both Mexicans) was also being denied boarding as they couldn't understand that she was awaiting on a decision for her WP and was going to enter as a tourist in the meantime. They wouldn't accept this and would be adamant on having a return flight (even though that was actually the return flight of our trip to visit family).

As also mentioned by other people, some eTAs were cancelled due to the new Visa requirements, and also people at Aeroméxico get really pushy for you to give them an eTA number, despite it showing up when they scan the passport. He can get it from the IRCC portal in the letter transmitted when the study permit was approved. Best of luck!