How can I prevent my small neutered male dog from being mounted almost every time he meets another dog?

The title says it all, really.

My little dachshund mix gets harassed when he meets other dogs (even females, but less often). The other dogs sometimes lick his pee, and unfortunately their owners are unable to stop them from mounting him.

I step in and protect him, which he gratefully accepts.

When we walk past male dogs, he immediately goes berserk, and it's even worse with unneutered males. With some, just eye contact is enough and he barks and pulls in that direction.

I can stop this behavior and redirect his focus back to me, but I would prefer to spare him this stress and encourage him to be more confident. My little one loses his nerve easily (due to insecurity). He is only 2 years old and I have noticed with my big one that she has become quite confident herself now at 4 years old and after many good encounters/meetings with confident dogs.

But because my little one is always being mounted or otherwise harassed, we don't get a chance to practice good encounters.

He comes from a kill shelter, he's been with me for a year, and we've already accomplished a lot.

First and foremost, I just want to help him become more confident!