My cat is only about 3 years old and he acts normal around everyone he knows or meets, except one person. When he first met this person, he acted like he usually would, but just recently he's become aggressive when the person is around, even if we are not provoking him in anyway. He would meow loudly if you moved closer to him. This all happens when this specific person comes over, and I find it weird because he never used to do this. Also I was wondering does him not being neutered have anything to do with it? (He's an indoor cat so I didn't get him neutered.)
You are certainly not alone in your struggle with a cat that is afraid of or aggressive toward one person, and this behavior is often a reaction to stress. Your cat may have been frightened by your guest or your guest may play too roughly with him, for example, causing stress and the aggressive behavior.
There are many things you can do to help get your kitty used to your guest and to help reduce any aggression there may be toward that person, from using cat pheromone products to changing your actions (and your guests actions), all of which are outlined in my response to this person whose cat is terrified of her boyfriend.
In addition, any time a male cat remains unneutered, there is an increased likelihood of aggression and other behavioral problems such as territorial marking/urinating behaviors. Also, even though he is an indoor only cat, not having him neutered increases the chances that he will roam far from home in the event that he does get outside (even accidentally), putting him at risk for many feline diseases. Male cats that keep their testicles also are at very high risk of developing testicular cancer.
Neutering your cat would be wise. No only will doing so potentially help with the aggression problem you have been experiencing, but can also potentially extend his life expectancy by 3 to 5 years!