My lesson from my reactive dog to you....
None of matters. The bottom line is you have a reactive dog that has several "bites" under his belt already and it's ultimately your responsibility to make sure it never happens again before you get sued and your dog gets euthanized.
While it is extremely infuriating for an off leash dog to bound up to your on leash dog, the only way you are going to stop this is to seriously up your obedience on your dog.
LEAVE IT! HERE! HEY YOU COME GET YOUR DOG RIGHT NOW! LEAVE IT! HERE!
LAT and BAT for behavior modification. Increased obedience so he knows only good things come from you and the way to that is ignoring that dog. And use whatever it takes, whether it's a clicker or a prong.