Guarding? Wary? What?

You could do a few things to maybe help him get past his wariness and guarding, which doesn't sound extreme.   At home, I let my dogs alert me about a stranger or doorbell or knock but then I say "enough" which means, I've got it.  Pretty much what you're doing by saying leave it. 

 For out in public, I might take him to a not too busy public area where you can sit with him and do some desensitization training on strangers.  Work at the level where he shows no reaction and never push him too fast.  Reward him for any calm/nonreaction to an approaching stranger.  If you have to start out from a long distance, so be it.  It helps to have a helper who is a stranger to your dog but knows what you are trying to accomplish.  You can find the entire process in many dog training books and articles. 

 Those sound like good ideas. My sis and dad wanted to take Simba to the city(NYC) in a few weeks again, he seems to be fine in crowds. It's more just when it's a couple of people.

Is it a bad idea to take him to the front door when he barks when he hears something to show him it's fine? 'Cause that's what I've been doing with him. I'll look into some articles on the net though, if you find any that'd be helpful feel free to link me. :) 

Adolescent and young adult dogs can suddenly become fearful of things that they previously accepted:

 http://www.diamondsintheruff.com/fearimprint.html

Jackie's suggestions for dealing with his reactivity to strangers are good.  Learn to play the surprise party game: http://www.youtube.com/watch?v=uFiZkAG_1JE

 Thanks for the links! I shall start that surprise party tomorrow with him, it totally makes sense.

Given the age and unknown genetics, I would not force the issue, meaning I would not start "flooding" him by constantly trying to expose him to strangers.  Instead I would go about business as usual, and work in the Look At That (LAT) game.  There may be variations but the way I use it is basically I mark (in my case click) when the dog takes notice and then reward.  This tells the dog that 1) it's OK to look and be somewhat alert, 2) you get rewarded for just looking and NOT immediately reacting and 3) stranger = treat = not so bad after all.  In general I would keep all training positive and motivational, and work on building his confidence in himself and trust in you to protect him.  Combine that with the LAT game, keeping strangers at a distance while he is uncomfortable, and giving him some time and space to mature, and I think he will be OK.

Liesje

Given the age and unknown genetics, I would not force the issue, meaning I would not start "flooding" him by constantly trying to expose him to strangers.  Instead I would go about business as usual, and work in the Look At That (LAT) game.  There may be variations but the way I use it is basically I mark (in my case click) when the dog takes notice and then reward.  This tells the dog that 1) it's OK to look and be somewhat alert, 2) you get rewarded for just looking and NOT immediately reacting and 3) stranger = treat = not so bad after all.  In general I would keep all training positive and motivational, and work on building his confidence in himself and trust in you to protect him.  Combine that with the LAT game, keeping strangers at a distance while he is uncomfortable, and giving him some time and space to mature, and I think he will be OK.

This also makes sense. So I'm clear on what you mean, if he catches attention to something, lets say a person walking if he isn't barking or hackles aren't raised I click and give a treat? Do I make him have eye contact with me first before giving it to him? Or do I allow him to keep an eye on the stranger? 

 My understanding of the LAT game is you click the second they look at the other thing. The click should automatically cause him to look at you anyway for his treat.

Kali is my most reactive one and she LOVES this game. She practically gives herself whiplash to get her click then treat. I still haven't managed to change her reactivity if I don't have treats though...

Yes, you click the very first glance at, in Simba's case, another person. Before he reacts, or has time to think about reacting. It's the looking that you are rewarding. You don't wait for him to look back at you, he will learn to do that anyway when he realizes that he'll be rewarded by you. It's a fabulous exercise because it combines clicker training (operant conditioning) to teach good manners, while also utilizing classical conditioning (counterconditioning and desensitization) by changing his emotion toward the strangers. One of the best exercises there is, IMO.

What the previous two said.  The LAT game is not for teaching focus, that should be done separately (and at this point probably in situations that aren't as uncomfortable for him, since you want his focus on you to be very safe and rewarding).

 Okay thanks guys! I'm going to start practicing this when he gets up from his nap. :) If anyone else has any advice as well, feel free to share.

I'm honestly not TOO worried about his reaction, it's just a bit odd because I'm not used to the whole puppy maturing, being cautious of things again for a while type of thing.