Gremlin just can't figure it out

Gremlin is a shy cat who is up for adoption. He is in the house with me because he is the designate "chew toy" for my young cat Oliver, to keep Oliver from harassing the older cats with his constant young-cat need to play. Gremlin is very personable, but he only likes to be petted when he's eating or is sleeping with me. The rest of the time he shies away. I can practically step on him and he won't move. He talks back when I converse with him. But reach out a hand and...whooooosh! He's off.

Which has made it a bit difficult to teach him to use the cat door to the catio, which I have put up now that it is too cold to have the window open all the time.

If he is waiting by it, and I go to push it open a little way for him, he dodges away. On the few occasions I've managed to hold it open for him (by looking the other way and ignoring him for far too many precious minutes of my day) he has managed to push back inside once he has gone out. But he doesn't comprehend he can push to go outside.

Rose, Bear, and Oliver have the door down. Fluffy gets it when he wants to. Pickles and Nellie don't even try, which is a pity because Pickles loves the catio.

So he just wistfully looks out the window.