Moray was neutered last month when he'd just turned 5 months, and he was fine. Historically we've always previously waited until our cats were six months old, but it seems that thinking on neuteing is changing, and early neutering is becoming more and more common. As Yul isn't going outside at present, and you dont have other cats, as long as he's not being too rough with Billy then I would say you can decide what you feel comfortable with.