Aggression is how cats keep themselves and their territory safe. If the cats (both) are not fixed their hormonal levels cause them to fight. Fixing a cat can sometimes lower their hormonal levels which reduces the aggression. Next, introduce the cats to each other slowly. Switch litter boxes so they get each other's scents. Feed them both close to the door that separates them so they associate eating without the aggression. Be sure to give them both equal time with you. Take out the kitten regularly and put your cat into the room where he had been. This allows them to get used the area, leave their scent, and get used to the other one's scent. This way they slowly accept the other cat because they can smell him in the house and the area where you keep them. Be sure to keep an eye on them when they are together, until you are comfortable that they won't fight. Please take note of the fact that an unfixed male will spray inside your house to mark his things, especially once the other cat has been there. Fixing them can prevent this from happening.