In today's world, letting your cat outdoors for any reason or any length of time is akin to playing Russian roulette. One of these times, its chances will be all used up. Outdoor cats are at risk for injury or death as a result of disease, other animals, poison, sadistic people, animal "bunchers" who collect strays and outdoor pets to sell to laboratories, cars, foul weather and a host of other dangers.
Reputable authorities speculate that outdoor cats have an average lifespan of two to three years, versus their indoor counterparts, who frequently live to be 15 or older. We have domesticated our pets, and as such have a responsibility to take care of them and look out for their well-being. Your cat may look longingly out the window as through he wants to go out, but the bottom line is that this is not safe. Creating a stimulating environment for him inside your home with trees, or toys and giving him lots of attention and exercise will ensure he has a full and enriching life, while remaining safely indoors.