My 9yo has several wooden practice swords. In fact, we just got back from a ren-faire where he was taking lessons from both a swordmaster and his journeyman.
He's also in Aikido, although he's not ranked high enough yet, he'll be learning swordwork in the next year.
In my experience (ex-military) the people who are the most dangerous, are those with no training. Those who have no respect for weapons. Those who don't know how to use them; whether they're projectile or edged or explosive.
The people who blow themselves up with fireworks goofing off, who shoot themselves and their friends in livingrooms goofing off, who go out hunting drunk... these people have no respect for what these weapons can do. It's like an unlicenced driver deciding to go joy riding. People get hurt. They're PLAYING. Which is not what one does with weapons. Learning and practicing can be very very fun... but you don't play. You don't goof around.
But you put a kid (or adult) in lessons; where they learn how to use and respect those 2000lb, 150mph battering rams... blades, pistols, rifles, explosives... and all of a sudden the goofing off stops. There are still accidents. The best driver in the world can crash. The most accomplished marksman can forget to clear their chamber, there can be faulty material in the firework... but they become accidents. True accidents. Not careless negligence stemming from a total lack of understanding as to what is safe.
People like power. Whether it's the power to sew a broken body back together, or light up a mind, or provide for their family, or win a competition, or write that perfect paragraph or score. Weapons are one kind of power. Money is another. Education is a third. There are DOZENS of kinds of power that people relate to and seek out. They make us feel alive. And secure. And darnit! We're going to be the best we can be with them. Whether it's professional (teacher, doctor, preformer, soldier), or for the pure joy of it (moms, personal passions/pursuits).
If your stepson in naturally attracted to the kind of power that weapons bring, get him in classes. Not to do so is dangerous. Because that attraction won't end just because it's not an attraction you share.
A kid who likes guns, trained in their use, may become a soldier. Or a hunter. Or a historical reenacter. Or a competition shooter. Or go into local or federal law enforcement. Or simply view shooting as a hobby.
Or they could be the dumb kid who sneaks his friend's stuff and shoots themselves because they've never learned to handle a weapon with respect. Or the dumb kid who (responsibly) takes the weapon somewhere safe / secret, and breaks their own nose because they fire a pistol like you see in the movies (kickback on a sideways held weapon either breaks your nose or your mouth).
Swords and knives, in many ways, are more dangerous than pistols and rifles because those only shoot in one direction. Big ole knives flung around, on the other hand, cut in every direction. There are numerous ways to learn to use/respect them (fencing, sword fighting, certain martial arts).
A person doesn't just never use power tools because they're dangerous. A person learns to use them safely. A person doesn't just not learn to drive a car because it's dangerous. A person takes lessons.
Can't highlight it strongly enough: Lessons. Pronto. Like last week.