I’m calling a bad combo of too much energy, inappropriate coping methods, and now stressed.
Easiest thing to try (aka, what I would do, in order): Handle the stress, handle the energy, handle the boredom.
Stress: First, intro some cat soothing pheromones around the house, and go buy some cat grass (catnip). Put some catnip rubbed on favorite toys, find a place where you don’t mind him rubbing and gnawing on, and rub catnip on it too. Refresh the catnip on the gnaw spot and the toys every day, and play with him when you do it. Leave that going for a few days.
Energy: After about 3-5 days with the catnip and the pheromones, start to tackle the too much energy part. Wear him out. Lots of kitty play sessions with laser pointers or cat toys. Try to run him ragged at least twice a day. Don’t stop til he starts breathing with his mouth open, or he stops chasing the toys. If he seems the type to like playing chase, then chase him around the house yourself. If not, tie a string to one of your shoes, or dangle it behind you, and see if you can get him to chase you around the house. * If he tries to nip you while playing, then drop the string/toy/pointer and walk away and ignore him completely for at least 5 minutes. *
Coping: Hopefully he’s gotten attached to his nice catnip smelling new gnaw spot. If you can find more than one place for him to gnaw on, that’s good too. Set them up the same way with catnip on them. Get a cheap-o spray bottle, fill with water, and keep an eye on him when you’re home. If he nips something or gnaws on something he isn’t supposed to - spray him! If he nips you, say OW, then disengage as quickly as possible and ignore him for 5 minutes. If he nips a guest, spray him!
I had a stress-biter who stalked people’s ankles, and managed to get her back on even keel after about 3 weeks. This does take time, but it does work. Remember to keep up with the catnip and pheromones and playtime even when you start punishing him for the bad behavior, so he doesn’t get all stressed and sulky on you again.