Hello and welcome to COL
Many dogs feel anxious when they're on lead and are approached by other dogs, off or on lead. I think it's probably because they're in a situation where they can't display normal dog to dog meeting and greeting behaviours because they're restricted by being on lead. What's Lenny like when he's off lead and meets other dogs? Is he more relaxed?
One of my Cockers, Louis, is fearful around other dogs since being attacked by another dog when he was a puppy. He's gradually becoming more confident off lead, but on lead he's very much how you describe Lenny. I try to keep him off lead as much as possible but sometimes I have no choice but to have him on lead.
I haven't found a solution to keep him completely calm on lead but there are things I do that can help him not to become fearful and aggressive. Some of them are fairly obvious -
I try to avoid other dogs when Louis is on lead. If that means I have to cross to the other side of the road or take a different path then that's what I do before the other dog is close enough for Louis to become fearful. In my mind it's all about finding ways to help Louis not to get into that state of anxiety where he loses self control and starts to bark and lunge.
I try very hard to stay calm and confident because that also helps Louis, but it's really difficult when I have another dog owner verbally abusing me because Louis has barked and lunged. Luckily that doesn't happen too often, but when it does it's horrible.
If Louis does the smallest thing correctly and calmly (like just glancing at another dog but not reacting, or stopping and staring for a few seconds then relaxing a bit and walking on) I quietly praise him and reward him with a treat so he can begin to learn that not reacting badly is a good thing.
Hope that helps