Become an active participant in your son's firearms fancy. By doing so, you will also be involved in why he has a firearm.
If you pay attention to his hobbies/personal life, you may be able to ascertain if his purchase is just a new found hobby, or something sinister. For instance, look into who he hangs with and if he belongs to a gang. If he belongs to a gang, notify the police. If he just likes to go to a firing range for enjoyment, no problem. If you live in the inner-city and there are no firing ranges in the area, I would definitely be suspicious as to his motives.
On the "assault weapon" comment, any item you assault another individual with is "technically" an assault weapon.
However, as Willipete, said, the military labelled rifles that can be toggled between semi-automatic and fully-automatic as "assault weapons." What your son has, is a semi-automatic, clip-fed rifle.