Maybe just for a half hour to start with, then slowly increase it by 10 minutes every other day or so. You can let him wear it longer when he seems to forget about it.
Maybe give him a treat when you first put it on, eventually try to engage him to play and move around...but that will take some time before he gets used to wearing it. I really don't think it will take too long.
Pretty soon, if he enjoys going outside for the fresh air with it, he will look forward to it. We used a harness/leash for out cats when we moved from WA to GA. We had to stay in an RV park for 5 months until our house closed and we could move in. If the kitties wanted to go outside, we had to use the harness and leash. They all enjoyed being outside so much, they barely paid any mind at all to the harness/leash.
ps...after a while, I'd also let him drag the leash around so he can feel it tug on him. I had one cat who would panic if he felt the leash pull on him too quickly.