This was my advice as well. He needs PROOF. Not just an ultrasound picture but an actual doctor confirming it in front of him. He needs this before anything else in my opinion. So far all he has is her word and the ultrasound.
ETA: Until he has this he should act as if he is the father. He doesn't believe she slept with other guys at all, but it is to protect him in case she is lying about being pregnant at all. Like I said I have known girls who have done this in the past ... like more than one hands' worth.