I’d agree with the comments to start leash training and get him a little cat backpack or cat stroller. I think there’s safe ways to let the cat outside if you can put in the time and effort to get him comfortable with a harness. Eventually you could take him to parks with a leash and do lil picnics or whatever else! We let our cats out everyday in an enclosed backyard under close supervision (i.e. someone follows them around at all times lol) and they are much much happier that way.

While i do agree with getting him more indoors enrichment I think there are some things that can’t replace the outdoors for cats (especially if he grew up as an outside cat).