Do NOT, for any reason, put the cat outside! There are too many dangers out there and he will not know how to defend himself from roaming dog packs, traffic, feral cats, starvation, coyotes, bad weather, poison, snakes, or people who torture and kill small animals for 'fun'.

If you're going to get rid of the cat, it's best to take him to the vet and euthanize him right now, rather than put him outside where he will almost certainly experience a short life and brutal death.