He's had "Jacky" on several times now, usually done up very loosely. Each time Jacky goes on, we go outside. I'm now able to hold him by the ring at the back of the jacket to prevent him jumping down. I did put him on the floor, briefly, but he started backing up. I could see that the jacket was too loose to allow him to continue, he might have got his paw stuck in it.
Today I did the jacket yp reasonably firmly. He was OK with it while we were outside and he was on my lap. He was allowed on the floor briefly. We came in, and left the jacket on. He tried to walk, but clearly didn't like the sensation. I remember what cats are like when collars are first put on, so I wasn't orried,
He started trying to back out of it, walking backwards, backwards, backwards. He sat down. He lay down. We gave him treats. He wouldn't bend down to get them.
In fact, he tolerated the jacket a lot better than I thought he would for his first full go. After 5 minutes, as recommended, we removed it and made a fuss of him.
We'll continue to build up gradually. As soon as I am sure he is happy moving in the jacket, we'll put a lead on... and then we'll progress to him exploring outside.
The key thing is that we want him to associate Jacky with Outside.