Feed him his breakfast in the opening of the door? If that doesn't Scare him too much! My Guys hate their old travel cage, loved their new bird Pouch from celltei because I let them associate it with only good things. They love it less now that I have been on a holiday but still have no problem sitting on it and we are working on going inside again. I use clicker training, click+treat when they look at it, then when they sit on the edge, then when they put their foot inside etc. I put their favorite treats and toys inside as well. With Fëanor I also let him sit on my hand while he munches on Some treats inside the carrier.
Perhaps placing it on a flat surface and offer several treats in the doordat during the day? Then as he readily takes them, move them a little more to the inside of the carrier. Not sure if you would be able to get him in in time for the vet appointment, but it might help a Tiny bit!