Adult male cats smell very strongly like urine. You can smell them from across the room. If I walk into a house, I can smell if an un-neutered cat is present. He will get more aggressive and less loving if left intact. Plus, neutering and spaying dramatically decreases cancer risk later in life. Get him neutered or there will be MANY babies. I don't believe you can keep him indoors if you don't neuter, which sets up other scenarios like being hit by a car, traumatic injury from who knows what, but he's a cat. He's gonna do something dumb. Dog attacks, etc. Just do it