A normal, well adjusted pet can almost certainly be trained to use a pet door. Generally, pets are so happy to be able to go outside that training is a breeze. However, your pet doesn't know what a pet door flap is at first so he will need to be shown how it works.

Training isn't hard. There's only one mistake to avoid in training: Do not frighten your pet!  He may associate his fright with the pet door and not want to go near it again. Probably the most common way to frighten is to shove him through the pet door. Don't!

Coax him through with a treat; praise and reward him for going through. Never scold or punish.

Here are a few more specific ideas:

•   Hold the flap open for him all the way at first; then less and less so he gradually gets the idea of pushing against it to get to the treat.
   

•   Try removing the magnet for a while so the flap is easier to push. The Endura Flap based pet doors allow for all magnets to be removed to facilitate training if needed as do all Hale Security pet doors. You may  be able to remove the magnets from other flaps also though you may end up damaging the flap in so doing.

•   Remove the flap altogether temporarily while the pet gets used to the idea of coming and going in that particular location. Or tape the flap up out of the way.
   

•   Electronic pet doors all make a sound when they activate that can spook a pet. Try disabling the electronics for a week or so while he becomes accustomed to the flap if the sound bothers him at first.
   

•   If you have multiple pets you're lucky. The first to learn will demonstrate for the others.
   

•   Don't get the absolute smallest pet door he can barely squeeze through. Give him a little elbow room.
   

•   If you're planning to use more than one pet door, have them all use the same flap if possible for greater consistency.
   

•   When you first use a locking cover or when you first lock the flap shut, introduce the pet to it slowly rather than to let him learn 'the hard way' by running into it.