I have a weimareiner that is the exact same way. Honestly, it has never changed with him. We can bring other dogs into the yard but he is never very friendly and if they get to much in his space he lets them know. The only time that he has actually fully gone after a dog was when a loose dog nipped at our other dog and he didn't take that well, although he didnt hurt the dog that I know of he just chased it really far away. Sometimes it is hard to walk him, I dont know if you are familiar with weimareiners but they are big too.. mine is 75 pounds but when I know a dog is around I grab him close to the collar so I have a good hold of him until we make our way past the other dog. Im not sure how aggressive your dog is with other dogs, mine I was just able to manage because it only went bad if the dog made a move like nipping towards him or my other dog. He is 12 now and he has calmed down a bit but still doesnt like unfamiliar dogs, we never did anything to change the behavior other then let him know he was a bad boy, we just learned to deal with it which sometimes was very inconvenient. Maybe someone else will have some advice to change the behavior, I can just say I have a dog just like it.
Be really careful though if you dont feel like you can hold him down.. I know one time my dog took off running towards another loose dog in the neighborhood and I literally went flying, I was completely off my feet but I somehow got lucky and landed it. My mom was also walking him and wasnt completely paying attention and he pulled the wrong way her arm couldnt support and she fell right down so just be careful.
"It is not enough to know how to ride, you must also know how to fall."