I commend you for being concerned and wanting to do the right thing.
As others have suggested, 17 is on the old side for introducing a young person to shooting (remember, the Army takes 17 year olds and gives 'em machine guns). I concur wholeheartedly with others who advise that you have some range time together. However, since this isn't your passion, you may also want to look around to find an adult firearms hobbyist (family friend, work associate, at a local gun club, etc.) who can mentor him.
My parents never owned firearms. I was blessed, at 13 (I'm 47 now), to find a mentor who was a very well-financed collector of military firearms. The knowledge and respect I learned from this experience was sooooooo beneficial as I grew into a young adult. My first rifle was a present, from my parents, for my 14th birthday (it was the first gun in my parents' home and I still own it today). Shooting and collecting has been my passion ever since (click on my Web page url below). I just recently introduced my daughter to shooting at 12 years old. She's doing just fine with it.
Since he's the shooter, let him
take you to the range and show you
the ropes on safety, range procedure, and marksmanship. This will give him an opportunity to demonstrate whether he respects firearms or is reckless (kids who respect firearms, respect other people and their property
This shared experience with firearms will give you additional ability to talk with him knowledgeably about firearms and more credibility when it comes time to approach him and try to impart some guidance.
The intense interest, in and of itself, doesn't concern me. Something that you might seek to influence is the way he talks about firearms. Some non-confrontational food for thought that you might share with him is:
Today, there are no crass "jokes" about firearms. What would have just been seen as "shooting off at the mouth," when you were his age, can get you arrested today. Having a record (i.e., making a threat about firearms) can make it difficult to purchase and own firearms (which he obviously will want to do someday soon).
Another thing that loose talk gets one who owns firearms is: burglarized. All your guns gone and a no-expenses-paid trip to Home Depot to buy all new locks for the house.
Owning a firearm requires responsibility with the mouth, as well as with the hands. Allowing him to earn the privilege to have a nice .22 rifle of his own at home, might be motivation for him to demonstrate to you that he can talk responsibly (and keep his mouth shut in public) about firearms.
One thing all firearms enthusiasts fear intensely is the government taking away our right to own guns. People who spout out irresponsibly in public play right into the hands of the gun-grabbers. Firearms enthusiasts who care about their hobby work like dogs to be goodwill ambassadors for our hobby to the voting public (I'm doing that right here).
Here's a pic of my daughter firing an original (not a replica) Pattern 1853 Enfield-Rifle musket. The Enfield was the second most common rifle used during the American Civil War and this one is that old. Whaddaythink that she will have a different appreciation than her classmates for our founding fathers, Abraham Lincoln saving the Union during the Civil War, etc., when her 8th grade class studies US History next fall? You betcha'.