You can't ignore the behavior, because you're leaving him to find his own confidence, and as he grows, he will not rely on you for what he should do, he'll do what he thinks is best. Or he may grow even more scared and not develop any confidence. Either way, it'll grow more hassles for you as the owner.
Does he try to hide behind you or get away from you too? If he's hiding behind you, this is good, because you can bring him out and show him it's ok and that you're not setting him up to get attacked by another dog. If he's retreating away from you too, that means he doesn't think you can protect him.
When I first got Logan he had been bitten twice at the "breeders" and he had a fear of new dogs. When Ricca threw her feet at him in play he screamed and ran. He didn't know me yet, so he didn't run to me.
I took him for a lot of walks, and when he showed signs of flight when another dog was approaching, I would pull him into a close heel, have him sit, and I would wait and see what this other dog was about. I would go say high first, and then invite Logan in. Never allowing him to go first. Every experience was a positive one, and he got over his fears quickly. He was also unsure about new people, so I did the same there too. I would tell the person to baby talk him and invite him in while they crouched down, and Logan soon learned "all people are really nice!".
I don't let him think for himself about other dogs. If the dog doesn't seem like it'll be a positive experience, I keep him focused on me and we keep going. I turn his attention away from this other dog. If the dog seems ok, I meet first, then Logan can.
I didn't get him until he was 12 weeks old so he was already missing some key socializing. But with me taking control and introducing him to it and being his "back up"... he quickly learned that he could trust my judgement.
Now I have to hold him back from wanting to meet other dogs, because his confidence has grown so much.
You have to be confident, and use your dog sense about if a meeting is a good idea or not. You also have to display that you can protect him. If a dog is coming up off leash, place your pup behind your knees, and use confident body language to deter the strange dog from approaching. Your pup will watch all this and will see you as pack leader. Point your finger, put one leg forward, say "ssshhh!" or "get!" to keep the off leash dog at a distance. Your pup needs to feel safe with you, and that will build his confidence and loyalty.
Eventually he will try doing this on his own, don't allow that either. He's not the one in control, you are. When you see a dog attached to a human who's coming towards you, ask them from a distance how their dog is with puppies. You meet the dog first, that display will boost your pups confidence.
When no dogs are approaching, he's to ignore them. Take some treats, practice "sit", "down", and things like that to keep his attention on you. When he surprised, like when the dog came out of the brush, pull him to you and make him sit and watch quietly. No barking, don't baby talk him about how it's ok, just distract him from what just happened because it's not worth panicing over. When he barks, tap the leash, say "No barking!"... no "Ssshhh baby it's ok" None of that.
Baby talking them through a situation feeds the negative behavior. Ignoring it and not stopping it feeds insecurities and makes for a dog that isn't easily distracted and it inhibits training.