Pup is misbehaving on walks, unsure how to correct his behavior

Hi there! I have two dogs, one is 6 year old Ziggy (Lhasa Apso/Shih Tzu mix) and the other is 2 year Neo (Shih Tzu).

The dogs typically go out in the fenced in yard and get a daily walk with a dog walker. The dog walker went on vacation this week so I’ve been walking them. Neo’s behavior has been out of control on walks. He literally wants to attack every squirrel and dog he sees. Yesterday on a walk, we saw a bigger dog and Neo went wild. He started barking and biting his brother’s ear when he got frustrated that he couldn’t get to the other dog. I’ve never seen him display this behavior before. The dog walker has never mentioned any issues with either dog. Neo is sort of shy so only tends to misbehave with people he knows well.

My question is, how do I correct this behavior when I see it? I’m not opposed to a shock collar when used properly. I used to use it mostly to beep him but haven’t used it in quite a while. With this new behavior, I’m considering bringing it out again.