A dog and cat dilemma

My dog and cat are literal best friends. Play together, sleep together, my cat won’t even drink out of any other water bowl unless it’s my dogs water bowl.

Last night my cat (6months) figured out how the doggy door works. I don’t want him to be able to go outside and I’m trying to figure out something I can do to deter him from wanting to go through the doggy door.

I’ve had a bad experience with putting a collar on him, even though it was a breakaway collar, I came home from work one day with it stuck in his mouth and he was bleeding a lot. I really don’t want to put a collar on him again.

I’m trying to find some sort of deterrent so maybe he won’t go in the room that has the doggy door in it at all.

Important mentions…. My cat LOVES water. Loves to swim and sit in the sink. So spraying water doesn’t really do anything. He also doesn’t care about citrus deterrent sprays-they don’t work. I have seen the electric doggy doors that will only open for the dog collar but the problem with that is 1, expensive and 2, might not work with my doggy door because it’s a hole through a wall. It’s not like part of a sliding door.

Any suggestions to keep a cat out but a dog through a room (room has no doors)

EDIT:

The reason I don’t want my cat outside is because my backyard is not fenced and it leads straight to a very busy road. I just wouldn’t want to risk it.

I have decided against the electronic dog door with a collar because I do believe my cat would just slip by. I am just leaving the dog door permanently closed and manually letting my dog out every few hours other than our usual walks. I travel a lot for work so that’s why I wanted to find a different solution because now I’ll have to find a full time pet sitter that I trust. It’s really not a big deal tho and I was always willing to do that.

I do have a harness for my cat and I have been slowly trying to get him used to it. Today is the first day that he wasn’t trying to take it off of him. I haven’t let him outside with it yet (on a leash) because I want him to be comfortable with it on inside first. I also got a very tall gate door for my hallway so my cat stays on that half of the house during the night so I can open the dog door during the night for my dog. It literally cuts the house in half so he has PLENTY of room, and it’s only from 11pm-7am. If I wake up earlier then I open the gate earlier.

Baby steps… eventually I’d like to fence my backyard and make it pet-proof but in the meantime this is what I got.

I would eventually like to take my cat with me while I walk my dog because they are so close and I can tell he wants to go outside, I just want to be smart about it.

I appreciate the feedback and it gave me lots of inspiration and insight on what to do next. Thanks!!