A couple of questions:
Where exactly are you located? There are a few knowledgeable folks on the forum who might be able to recommend some good behaviorists, but they'll probably be primarily US-based and I suspect you're not by the "neighbour" and "behaviour" spellings. @NeylasMom@greysmom
Is Finn food-motivated at all? Sophia Yin has some older but helpful YouTube videos about leash reactive dogs like this one and this one. I used the "look at me" command to train Sweep mostly out of her leash reactivity, but that girl would do just about anything for food, which helps a lot! Searching "leash reactive" or even "lunging" on the forum should yield some helpful posts.
This is trainable—thank you for sticking with him!