This was an individual specifically trained in lifesaving techniques for accident and emergency victims, and he should not leave an obviously injured person. His training prepared him for just this situation, and since he was the first person on the scene, he could literally make the difference between life and death. The accident victim with an obvious head injury could be suffering from shock, and the head injury symptoms might not show themselves immediately to the victim, or he might not understand what is happening, and what his symptoms indicate. The paramedic does, and should recognize the danger of leaving a head injury victim alone waiting for other help. There are those who might say that the paramedic had no duty to stop in the first place. This is true, since he was off-duty, and not compelled to respond to the situation. However, he did respond, and therefore, that makes him morally and ethically responsible to stay with the man and monitor his condition...

He chose to stop, and so, he should see the situation through to completion, rather than leaving because of the opinion of a patient that may not be fully cognizant or coherent. He could also observe the patient and give valuable information to the responding paramedics in case the man's situation deteriorated. It is his moral and ethical obligation to complete what he started. He obviously knows more about the situation and what could result than the victim, and so, he should use his professional knowledge and training to stay with the person until it is safe, and leave him in qualified and equally knowledgeable hands.
In conclusion, the paramedic's training and knowledge obligated him to stay with the man at least until other help arrived. The fact that he had more knowledge and experience than the victim indicates that he should use his judgment and stay, rather than simply taking the word of an injured victim, and leave.