(1) Train him so that he recognises you as his pack leader and then he wont feel he has to protect you - currently you are allowing him to be alpha dog, which is BAD;
(2) Socialise him with other dogs under supervised conditions - training classes will be absolutely ideal, under the eye of an expert.

So, training it is then! Call your vet right away!

How are you when you are with your dog? Are you the leader to him? Do you walk out the door first when you begin the walk? Are you in control with him from start to finish? Those are the types of questions that really help me when I walk my dogs. Don’t give them an inch for at least the first half. Near the end of the walk, I reward them by giving them a ‘key’ word like, “okay!” and I’ll let them have a long lead and let them sniff around. But the first part of the walk, I keep them on a shorter leash. I want all their attention focused on me. Sometimes I’ll even bring a few slices of bread with me to keep their attention. This practices discipline on their part, and leadership on my part. Dogs love to have a leader, it makes them feel safe. It is also a great mental exercise for them.

If he’s dog aggressive on walks, as soon as you see the dog coming, make him sit. Don’t tense, be very calm but assertive and firm with him. Don’t get frustrated during the process because he will pick up on that, and that’s not what he will listen to. If he gets fidgity, correct him right away. If you let him act up, it will only escalate into the lunging and barking. Hope this helps a little bit. Good luck.

A method that usually works is to get a small sweet tin and half fill it with pebbles or coins so it makes loud sharp sound when shaken and attach it to a length of string. Enlist the help of strangers (ie friends of friends) with dogs to assist you. Sit on park bench with your dog on shortish lead while the volunteers approach and walk past you, some with and some without dogs. As soon as your dog barks or reacts hold the string and throw the tin down hard on the ground in front of him, tug his lead and say no firmly (the string makes it easier for you to retrieve tin and will add to his confusion). Keep doing this each time until he gets the message and is hesitant to react to people, each time he doesnt react and is quiet give him a tasty treat and praise him. Do this while standing by the path as they walk by, and progress to walking towards and passing these people with their dogs on and off leads. Every time your dog reacts badly shake the tin hard straight away, tug lead and say no, the times he doesnt react praise him loads and give treat. Eventually he should ignore other dogs and if he seems about to grumble he will respond to just ‘nooo’ and reasuring commands & prompts like ‘walk on’ or ‘leave’ & ‘good boy’. Remember you are not trying to terrify him, the idea is he wont like the noise and will be unsure so he will look to you for reassurance and guidance he will soon associate barking at people and dogs is an unpleasant experience and not barking at them gets praise and treats.
Lots of fun and basic obedience training will reassure him that you are in control and he can relax, walks will become more fun for you both. Classes are a great way to socialise him plus you will pick up lots of useful tips yourself. Take lots of tasty treats like pieces of sausage when you take him out for walks and ask other doggy people to help you by giving him treats. Getting him neutered if not already done will also remove his need to challenge other males.

I still suggest you get the advise of a professional who can discuss with you the trauma your dog has experienced and his relationship with you. I wouldnt want you to do anything that could cause him more problems.
I do hope this is helpful - good luck

Rik