Pit bulls have been known to be animal aggressive. Even though you say that he does not attack unless provoked, I would highly suggest keeping him away from that situation at all costs. First, when driving with him in the car keep all the windows closed. This is for safety also, because you will be devestated if he jumps out in the middle of traffic and something bad happens. Secondly, do your best when walking him to avoid all other dogs all together. If one is approaching you cross the street if necessary or go as far around as you can. Do not get within leash distance of any other dog. Also, if you cannot hold your dog back, I highly suggest a choke collar or a prong collar when walking them. This also can be used to slightly tug (not hard) the dogs face away from any distractions (aka other dogs, cats, etc). This will keep their attention ahead and can be a good training tecnique for proper dog walking. Another thing to do is to make sure that you do not give him enough leash. You must be able to control your dog at all times when walking him.
My husband worked nonstop with our dog Bloo when she was a puppy. In fact, he still works with her. Her only problem now is getting too excited when people visit. When I walk her (I am a small female) and use our prong collar (when used properly it is extremely safe), I have no problems. I walk her every day too. I have only seen her get agressive with one other dog, another pit bull named Xena. Which is strange because she has met thousands of other dogs with no problem. They just both hated each other at first meeting. Anyway, I make it a point to avoid any type of confrontation because I don’t ever want to be put in the situation of having someone who hates the breed acuse her of anything.