Forgive my ignorance, but when was it said that the Haitian can't stop Adam's powers? I don't remember anyone saying it on the show.
At a certain moment (I tried to find the exact time, but I couldn't find it) Peter asks Adam why they were in prison if they were dangerous and just didn't kill them. In reply Adam awnsered that if they could, they would have done it. Keeping in mind the Hatian can stop powers, it suggests that Adam has powers you can't stop.
Next to that Adam also states that his blood has healing powers. It isn't something he does himself.
This can also explain the fact why Peter could survive the bullits without having to get them out before healing (remember Claire can only heal if she takes out the foreign objects in her body).