Maybe he is trying to drive other people and dogs away because he thinks they are to blame for any punishment he recieves as a result of his screaming.

The problem arises where a dog is being walked on a lead and another person or dog passes by and maybe your dog pulls towards them out of curiosity or other reason. Pups often go through a fear stage where they run up and bark at other people and dogs but don't actually go right up and attack. They usually turn and run away again with tails between their legs and they eventually grow out of it...provided the owner doesn't start reacting and teach the dog to get worse.

If you did anything unpleasant to the dog, no matter how slight, such as tightening the lead and maybe tugging on it a little or telling the dog off for pulling towards them, becoming anxious yourself etc...he will come to believe that he gets punished whenever other people or dogs pass by and this will make him want to drive them away quick when he sees them to try and avoid being punished again.
He doesn't realise he is being punished for his behaviour towards them, he thinks their presence alone is the reason.
If you start getting anxious when you see another person or dog approach, he will feel it and think you are being anxious about them and not about his behaviour and this will just reinforce his belief that they really are bad news.
If you tell him off, again, this does the same thing, he thinks you are joining in trying to drive the other person or dog away. In this case, you would actually be teaching him to become more aggressive rather than teaching him to stop being aggressive.

At his age, it might be very hard to change his view on these things but you need to start making the act of passing others a more pleasant experience for him. He needs to view the appearance of other dogs and people as a prelude to something good such as a treat or game with his fave toy. If he has a toy he loves, try to destract him from other dogs and people with that. If he does still have a go, keep calm and quiet but just hold firmly onto his lead but don't yank it or pull him about and keep walking at your usual pace.