Best option for vicious cat

Hi, welcome to TCS!
So as I understand, your cat has been with you for 3 years? He is indoor only?
How often do you trim his claws?
Aggressive feral behavior in cats include hissing, growling, followed by an attack. They only do it if they are scared or cornered otherwise they will run and hide. Cats dislike confrontation. Your cat may actually be rough playing. The constant shredding of furniture and boxes could mean boredom.
When you tried getting another cat, did you do a slow introduction?

Majority of the cats here are indoor only.
Those whose cats crave the outside are advised to get them outdoor enclosures. It could work for you too. But there are members here with success stories of converting outdoor cats to indoor cats. Hope they can give you their advise.
I don't think the shredding is done out of stress. My cats are indoor/outdoor, have lots of trees to claw but still manage to tear the occasional unattended cardboard box.
On the subject of the cat hurting you and your son. Have you tried hissing and blowing at his face whenever he does? My oldest cat was feral. I used this to make her used to my touch and did the hissing and blowing to tell her when her behavior was unacceptable. It took time and patience but now she's a pretty mellow girl.

So the cat doesn't like your son? Hmm, odd...

Does he have very different body language? I'm autistic myself and I have been told I have an odd or bouncy walk, and I'm generally uncoordinated. Maybe your cat's looking at your boy and misreading his signals, because the cat is used to regular human body language and is misreading this as a "threat"? Of course, it could be totally unrelated to the autism. Goodness knows that cats decide to like or dislike people for the oddest reasons. Maybe he doesn't even dislike your son, but is trying to "play" and just sucks at being polite about it.

I suggest for one thing that you should tell your boy to wear jeans until the cat thing can be solved--jeans, or something the cat's claws won't scratch through. Being ambushed by the family cat would be bad enough without ending up having to go through the bacitracin-and-bandaid routine with every incident.

Your son's right about the danger of coyotes. That happened to one of my cats, and I have never quite forgiven my mom for letting him out.

Let's see... Aggressive or nervous cats often benefit from Feliway. It basically smells like "calm cat" to them. That might be worth a try.

Are you sure you can't clip the cat's claws? I assume it is possible, since you managed to get Soft Paws on the cat. If so, you can probably blunt them enough to stop any further ambushes from resulting in injury to your son. Just use a human fingernail/toenail clipper--they work as well as specialized cat nail clippers. Or take him to a groomer? Might be an interim solution.

Tried asking your vet whether there is an antidepressant or antianxiety medication available for your cat? I know, it's odd--but kitty Prozac exists and has done some good for some cats.

How about tiring out the cat playing with him? Sounds like he's got a huge hunting drive and he'll find something to hunt, whether it's a mouse or your foot. Maybe wand toys and lots of interaction? Laser pointer?

Oh, and with attacking feet: Tiny used to do this quite a bit, though he never bit down or used claws, unlike your cat. I managed to deter him somewhat by simply not reacting--standing still and not giving him anything to chase. Cats are triggered by motion and if there's nothing moving, it's not that much fun. Obviously you couldn't do this if your legs weren't covered in jeans or something--you wouldn't want him scratching and chewing on you--but if you and your son could just make yourselves as thoroughly uninteresting as possible to pounce on, that might help.

Regarding shredding cardboard: I think he probably does it because it's fun. Do you know if he likes those cardboard scratch pads? My cat Christy loves them. She's the first one to claim a new one, she loves getting her claws into them and getting a good scratch, and she tends to just sit on them, hanging out.