I'm joining the chorus of people who have ND children and see my children in your description of your son. Mine are autistic and ADHD with a demand avoidant profile.
How often do you ask for his input on where you go? You might think wetlands are a great day out and he should be happy about it, but what if he would rather go to the little park round the corner, where its small, quiet and predictable? The easiest way to get my kids to go somewhere is to make them feel like their voices have been heard. If they'd rather go to the crap little park around the corner, that's where we go rather than a park i decided was better. We save the better park for the days they're regulated enough to cope with it. It wouldn't be my first choice but it's what they need, not what i want. Ultimately, they'd still be out in the fresh air running around.
Also he may appear to be fine at school, but he might not be. ND children are very very good at masking. The fact he's displaying this behaviour at home suggests that he is in fact, not fine at school, if he's coming home from school dysregulated. Don't rely on school staff to help you work out what's going on as well - most teachers have about an hour of SEN training and are woefully unprepared to identify or support ND children.
My children thrive when they're given autonomy - not always having things decided for them and then they're made to comply with it. He's trying to tell you in his 6 year old way that he needs more rest and more autonomy. I know lots of parents that are threatened by the idea of their child having the ability to decide where they go and what they do, but it's rather essential if you don't want to have to keep putting up with the "behaviour" that is more than likely him trying to let you know he's tired, overwhelmed and over stimulated and he just wants to stay at home with his toys.
Edit: i know you said he doesn't rest, but for ND brains, rest doesn't necessarily look like laying on the sofa quietly watching TV or reading. Rest can take many forms but it's essentially the person doing whatever it is that makes them feel like themselves.
The situation with him not being able to play on his own and ruining his brother's game - that could be equalising behaviour where he's frustrated and takes it out on his brother as the weakest person in the room. He needs more supervision and you need to step in when he starts to get dysregulated not wait till it's already happened. Some children just aren't developmentally capable of playing by themselves, especially if he perceived that his brother was getting all the attention from his parent.
Also look up declarative language and low demand parenting. It's changed our lives for the better.