I need help with my aggressive 4 1/2 year old neutered male Jack-Chi

by Deborah
(Carmichael, Ca, USA)

I've had my little boy since a small puppy and he has always had, what I consider to be a severe case of "small dog syndrome" where he becomes very aggressive towards them, barking very mean and if he isn't on the leash, he is so fast I can't catch him, he is a bad boy, I need help, new neighbors just moved into my apartment building with a Pitt/Corgi mix and OMG, now its large breed dog but pretty much eye level and my boy got loose last night, and although he didn't bite the dog and the dog didn't bite him, it still happened nonetheless, can you please give me suggestions? Thank you so much, I'm so desperate and at my wits end in this matter