It sounds like although he gets these violent thoughts he seems to have control when actually facing the person he is angry with. Some people may feel anger at a higher level such as wanting to hit that particular person but when think it over they will change their tune and react to the person in a mature manner.
As for how to tell if a person would act on their thoughts? I gather you do not know this person that well since you have been out with him once, few phone calls?
I would say if his thoughts become more violent, threatens others or you, starts to hit walls, throws things, jumps in his car drives off madly etc that could be a big problem. People have different ways and reactions so it is hard to pinpoint on how to tell exactly if they will act on their thoughts.
I have been in a violent relationship so whenever I hear someone who has thoughts like this I tend to be cautious. I'm not saying this guy will become abusive but if you plan on being friends or more with him, just be on guard. You don't want to end up with someone like this.
Here is a link in regards to warning signs of an abuser
28 Signs of Abusers