2 year old starting to show aggression or just a result of improper training (or not enough)

Hi. I have just registered after spending hours on-line looking at info and blogs about Pitbulls. I have a mixed rescue that I have had since about 4 months old. I had him neutered right away, he spends a few days every month in supervised play at doggie day care, and he has been great with people until recently. We had a trainer briefly but didn’t like his corrections, so we have tried to work with him on our own. He listens well, comes when called, sits, goes into his crate, jumps through a hoop and takes a bow. He is around 2 and a half now. When I walk him (which, I do everyday), he completely freaks out if another dog walks by. I walk out of the way and he is always on a leash but he jumps, pull and whines. Sometimes he even jumps at the other dog and snaps if he is close enough. This morning on a walk, he jumped and nipped at a person who jogged toward us. He did the same thing to a stranger who bent down to pet him a few weeks ago. He didn’t actually bite and wasn’t growling or showing other signs of aggression but of course, I am very concerned. I imagine that I have made some mistakes earlier on that are causing this behavior and I am hoping that it is correctable. Any comments are much appreciated.