I've recently attended a "Biker Down" course run by Kent Fire and rescue (HIGHLY recommend BTW) and the first part is all about what to do if you are first on the scene of an RTC involving a motorcyclist. The thing they stress a lot is your own safety, YOU are number one priority at any accident you come across.
Secondly, you are taught to access the situation, and I think, reading your post, you did this well. The fact he's sitting and on the phone is a very good sign, if he had been laying motionless on the ground with no one else around then you would have a right to feel guilty for not stopping.
Quick pop quiz for a Monday mornin, one question that was posed was "you arrive at the scene of an RTC, 2 people involved, one screeming at the top of their lungs, the other laying on the ground, silent, who do you go to first?"