You just described my son almost exactly! The only difference is that rather than being overweight, he is tall, thin, wears glasses, and has beautiful, flaming red hair. He's been picked on for being a nerdy "ginger" who has an "old man body" for years.
He was diagnosed just a month ago, but has carried the label of ADHD since he was 6 and epilepsy since he was 10. He's used to labels. He refuses the ASD label, but agrees that his behaviors are consistent with ASD. It's been a month and just last night he said that he might have ASD. Fine. But, we've already told him that the label is just a useful tool to have other people understand him. It doesn't change anything, he's still the same kid, but the label carries a meaning that other people understand so it's useful to us sometimes. So far, the label is for school use only by his request (he really, really needs accommodations for ASD), and he will share that label with others whenever he feels like it.
He has understood the social construction of rules, some of which really are ridiculous when you think about it, for some time. We have long been teaching him that even though sometimes they seem like dumb rules (when they are ones that are counterproductive or illogical they are really hard to tackle), he needs to go along with them in order to keep others from being upset. He needs to "play the game." If he does this, we tell him, life will be easier for him and he can focus on the things he really likes. That makes sense to him. Now we just need to help him make his behavior fit the rules. He's got a long way to go, but he's trying.
This is the approach that his counselor takes as well.
To get him out of the house my husband takes him to a game store to play miniatures games (Axis and Allies, X-Wing, Hoards). Every Tuesday and Wednesday he gets to spend a few hours with other guys, including some other fathers and sons who are reaching out the same way that we are, who are as obsessed about these games as he is, and he can have a few hyper-focused hours of sharing his his interest with other people without the expectation of small-talk or even "proper" behavior.
He has just one friend in our town, and he goes to church youth group once a month in another town. I wish he had those kids with him in school (they are very accepting and loving), but on the other hand his counselor said that it might be a good thing since they are untouched by the gossip about him because of his many behavior problems at school.
So very frustrating, I know. That's why I'm here seeking out people who understand too. You're not alone!