It's cruel to put a cat out at night unless it has a covered place to go to. My advice would be to pick him up, take him back to your house and keep him in for at least a week. One good tip is to smear butter on his paws, he will lick it off and in the process this will remove all trace of where he has been when roaming, he will take on the smell of your house, all over again and this should keep him with you. Good luck.