You aren't walking him at all, as a part of your regular routine?
That's the answer. When you get home, dark or not, taking him for a walk, or better yet a jog. 45 minutes. Ideally, you will walk (jog) him for at least 30 minutes in the mornings as well.
You got a very high energy breed yet he's getting almost no actual exercise. Sure, he's playing, but that's not the same. He needs workouts, essentially, to put it in human terms, not just walking from his desk to and from the copier and HR on the 4th floor. And not a casual stroll on his lunch hour. A true work out.
Yep. Play won't suffice.
I've rehabbed a lot of really, really messed up and dangerous rescue dogs and I've never had to use meds. I have had to put in a ton of diligent energy walking them though, and with proper walking technique: short leash, focused, in step, no screwing around.
My current dog is prone to total fucking psychopathy, and the only way to get his fuckery in order is to walk him for a very long time, on a short leash, at least twice a day until he's both physically and mentally exhausted.
He could run around and play with other dogs for hours and get out physical energy, but that wouldn't do anything to temper his psychotic face-biting, violent neuroses. He'll just nap for a bit and wake up just as much of a lunatic as ever.
Dogs respond really well to very predictable structure and discipline, so burning mental energy is critical, in a structured way, and on a schedule.
When my little dude starts getting a little Pesci, the key is to walk him at a brisk pace and get him into a focused flow-state, and employ a lot of strict boundaries at home, such as not being allowed up on any furniture, and doing regular discipline exercises like me putting a treat on the floor and making him wait in a calm state before being allowed to have it, if at all.
He has to be challenged to do the mental work of walking with discipline and holding himself back from what he wants (the sofa, the treat), and that mental work wipes him out and allows him to calm down.
He's never a perfect dog, he absolutely hates people coming into the house and will go absolutely hysterical if it's a man, but the rest of the time, he's a chill little affectionate guy as long as we reboot discipline-mode every few months to keep his demons at bay.
Structure, structure, structure, discipline, discipline, discipline.
If you can't do that with one dog, I strongly recommend NOT getting a second dog. A neurotic dog may benefit from a friend or may just transfer it's neuroses and then you have two difficult dogs.
Buddies have been both good and bad for my guy. He's been with me for a decade, and I've rescued a lot of old puppy mill dogs in that time. My dude is a guard dog, so having a fragile, old friend to protect means he's working 100% of the time. It's great, it calms him and keeps him focused, like puppy ritalin. However, it also makes him far more dangerous if he feels his charge is in danger. So another dog is always a complication, even if it is a net benefit.
There's absolutely nothing wrong with knowing when the job isn't right for you though, there's no shame in realizing that you don't have the capacity to be what this dog needs.