I think part of parenting the child/children you have, is picking your battles. And I agree that going to the park after school through a UK winter is often impractical and a bit shit. Even if it isn’t pouring with rain, it’s never going to be a hallmark movie experience.
My DS is now in his 30’s and wasn’t diagnosed until adulthood. Believe me, we tried, but apparently he just needed to try harder to sit still and concentrate, because he clearly had/has ability. But my point is that at your DS’s age, his thing was watching technical videos which used to be commonly included with household appliances like washing machines and vacuums. He would watch them over and over after school, much to mine and DH’s bemusement. Now that I have a better understanding of ADHD (I work in an adjacent field), I can see this was a form of sensory seeking, calming him after his day. I think your DS too is finding his outlet, it’s just that it’s not working for you.
So whilst I don’t necessarily think that sitting still on the sofa is right for your DS, I wonder how else he could meet his needs, that you would find less annoying? Ignoring the perfect parents, most of us asked our DC about their day, gave them a drink and went through their book bags, before using the TV to entertain them whilst we cooked dinner. It’s not neglectful, it’s parenting 101.
What I would suggest is to ask yourself honestly how much of a problem is the standing in front of the TV? Is it actively dangerous or just annoying? If it’s dangerous, then your best plan is to find another safer route for his energy. Accept that sitting still isn’t likely and find an alternative. Get an indoor trampoline, a pull up bar for a door frame, to be used in your eyeline only, or some exercise bands and let him wriggle away.
Alternatively, if letting him stand too close to the TV isn’t harmful, could you let him do it? Agree that it only until dinner and then the TV goes off until later, and he doesn’t get to stand in front of adult TV later on.
I also wanted to say that I get the panic that comes with the realisation that a condition like ADHD might be a possibility, and I’m not going to sugar coat it and pretend that it’s an easy ride. What I will say is that my DS now works in a job that gives him an adrenaline fix and that works for him. He’s also happy, sociable and in a long term relationship.