Sounds like uncertainty, and is fear reactive to people who aren't in his immediate family.
You can start out at extreme, and if he is under threshold still you can move him up faster than you can say with my girl.
First start with keeping distance with people, he can look alert. But if he is under threshold (which is what you always want him to be at) he should NOT be barking, growling, hair up, tail tucked, tail aroused, trying to run away, etc. But he can be watching him. As he watches them give him a treat and praise him. Eventually your distance will become shorter and he will still be under threshold, and eventually (if you stick to it, and have lots of interactions and go at his pace, could be months, years..) People can pet him and it will be a positive thing for him.
My dog has fear aggression to all strangers. Zelda is similar and so basically i am working on changing her response to unfamiliar people from one of fear to one of relaxed happiness. During this process i need to keep her safe and people. I am changing that emotional state of fearfulness which in lots of dog cases can and does lead to aggression. Thankfully Zelda gives lots of signals and calming signals. Which i do not want to stop, because it lets me know when its over her threshold and this is important not to stop, as it can lead to a unpredictable biter. Which is never easy to work with.. instead of trying to change her reaction to strangers, i want to change her entire emotional response (fear), because that is what is causing her reactions.. counter conditioning works for humans too! The more positive interactions the better. However, it's better no interaction than a negative. So if you are uneasy about being able to control the situation than leave. Better to be safe than sorry. And just so you know, my whole life had to be changed to ensure her safety from herself and other peoples. She is not even close to our goal, but improvement is still a step forward and that is all i ask.
Patricia McConnel, The Cautious Canine-How to Help Dogs Conquer Their Fears has really helped me, i suggest you buy it or borrow it from someone to read, a great resource. So is Sofia Yin.