My 4 month old lab is aggressive towards people when out on walks or when they are outside our gate/fence.?

I have a 4 month old black lab that is very aggressive towards people we p on our daily walks. He is also aggressive to anyone standing outside our fence. We are stationed overseas and were gone for a month. He stayed with a dog sitter that he didn't want to leave when we arrived back home from our stateside visit. The dog sitter stopped by today to check on him and my dog acted like he wanted to eat him. He was growling and barking. We were both afraid to see what he would have done. I do not know if this is from our being gone for a month or if he was like this already. He was already barking at per-by's on the sidewalk when he was in the yard but not to this extent. What can I do to stop this behavior now while he is still young? Getting a professional to ist me is going to be difficult due to the language barrier I am faced with. He is good with family and visitors inside the home. It is the innocent people we p on the street I am afraid for. Thanks for any suggestions.