First of all, you may not be correct that what he had was oral herpes. Lots of cuts and sores appear on the lip which are not herpes. Of course, even if so the orthodontist might have been better advised either to explain the lesion or cover it up, but that's another question. As for touching your lips afterward, I think that transmission from the lip (assuming that it was contagious) would be very difficult. He would have had to rub the lip and then go directly to your lip.
I think you ought to consider calling your orthodontist about this. I think it's more than likely that he'll be able to set your mind at ease more definitively than I can. It seems very unlikely to me that he'd work on you with an active oral herpes. Ask him what it was,
Best.
Dr. Rockoff
In my opinion, you could probably get it. But it's just a cold soar. It will go away within a week or so anyway. It depends though on how close he got. If he kissed you, I'd say you could probably get. But if it was just a dab or something, just clean your face off with benzothonium chloride or some other cleaning agent, not much would happen.