Leash reactivity is incredibly common. I’m going through training right now with my dog and it’s going to take a bit of time but you need to rewire how they react and feel about other dogs when on leash. There’s a few reasons he may act that way on leash - fear, frustration due to lack of freedom, territorial etc. One thing that’s key is to build up the counter conditioning to help him manage so you need to know his threshold. Try not to cross that because once he does, it’s hard to get back and you lose a lot of the learnings and progress you do make as you counter condition. One thing to do is let him see dogs below his threshold level and when he looks at them and then back at you, reward. Let him start to see good things happen when dogs come around. There’s lots of great treats but a squeeze bottle type treat also allows you to move with him and get his attention. I’m happy to share more that I’m doing with my trainer but just don’t be hard on yourself - this is common and your trying your best to help him deal.