He may have gotten smacked or spooked going through? We had a cat flap in the screen door that Pluto used to use. Then we locked it because the cats started getting out (go figure
). When the cats are outside in their catio, I started unlocking it again but Pluto seemed to forget how to use it.
With some high value treats, he quickly got back into the swing of things.
Try some hotdog or string cheese as a reward and I bet after awhile you'll be fine again.
One last thing - make sure that it's not something outside that scared him or is scaring him and I'm assuming that your backyard is very secure. We have a very safe high fenced backyard and I have windows on the entire back of my home facing the yard, so I can see Pluto at all times.