Kitty door

We just put in a kitty door in our laundry room door so the cats can get in there to use the litter box, but the dogs can't get in there to eat what is in the litter box.

Our kitten is adapting quickly and has used the door already on his own. However the 12 year old cat wants nothing to do with it. We tried luring him through it with treats, but he's more interested in getting away from the door than he is in getting the treats. I also tried just leaving him in the laundry room by himself after showing him several times how the door worked. I didn't want to leave him in there long, and went down after about 15 minutes and helped him through the door to get out.

Any ideas on how to convince him the door isn't going to kill him?

This is the model, and it says it's for cats/dogs up to 25 pounds. I was thinking of trying to prop the door open for a while. I'd rather not because I don't want the dogs getting any ideas, but if Max doesn't come around I think that's what I may end up doing.

I put in a cat door on my garage's side door for the strays and I had to tape it up for them to get use to it. Good luck.