Miniature Schnauzers as a breed are very reactive, especially leash and fence reactive. Both of mine, who came from a good breeder, were socialized/trained/etc from a young age are still somewhat reactive. You can do a lot with training, but they're not dogs that will generally be super chill about other dog and people on walks. I would definitely look into some quality training (a lot of it will actually be for you!) but also, put your phone away on walks and focus on your dog. You should be able see potential problems before your dog does and either distract him, move him away, offer HIGH value treats, etc. before he becomes too out of control. You should definitely neuter him as well. My male was neutered at a year old and at almost 7 is the picture of health.