My son had surgery for a deviated septum. they knicked his main blood artery while doing this. could this now cause him to be dizzy all the time?

Probably not: There isn't a main artery on the septum. There are plenty of blood vessels in the nose, many of which can bleed, but septoplasty typically isn't a surgery associated with much bleeding. If he did have a lot of bleeding he could be anemic, but that would be highly unlikely. He needs to see his ent, or another for a second opinion, and discuss the dizziness with them.