Yes, he could have some Aspie going on. He could also be right about the neighbourhood kids. difficult child 3 certainly was (on both counts).
The thing is, having a cool bike can be a conversation-opener as well as a tension-defuser. If difficult child keeps running, he will continue to feel afraid and to use running aas a tactic. He needs to learn the social skill of defusing, to be able to turn this around.
I wonder why so many of the neighbourhood kids came round at once. Not a good sign, in my opinion.
I also ride a "cool bike" (trike, in my case) around the town. It's a typical little old lady mobility thingie and there have been times when I've been out at night and there has been a group of drunken hoons getting aggressive. I know these same kids beat up people in the town. One night I heard one of them, on seeing me, yell, "Look at what she's got! I want it! let's get her!"
I had no way of outrunning them - this trike goes at 8 km/hr, top speed. A fast walk can outpace me. So I turned and faced them. As they approached I pretended that OF COURSE they had been joking. Never show fear - first rule. Take it all as a joke - second rule. Use gentle humour in reply - third rule. Try to avoid publicly humiliating them (except perhaps singly, if they really deserve it - and then, use humour).
So with the group of thugs, as they got closer, I said, "You think THIS is COOL? I've been disabled for 25 years, I never thought of disability as cool."
I answered their quesitons about my disability, about the scooter (making it sound less cool - "It has to be recharged a lot, it is very embarrassibng when it gets a flat battery and I'm still miles from home. You feel a right git sitting on this thing waiting for the tow truck"). Fairly quickly their general curiosity overcame any sense of "gotta have it". The main statement I made - "You can have one of these, anyone can have one. But they're fairly gutless. If you want this one, you've got to have the disability to go along with it. I could arrange the disability for you, if you really, truly want it."
And I say it with a grin.
But also, a big important factor here - I'm female. It is very different for a male, even a kid male.
Your son sounds younger than these kids, and there is only one of him. You could "walk" your son through the social interaction and in the process you get a closer look at these kids and work out what is frightening your lad. Or you could find a way to involve all the kids in something independent of the bike - a ball game, for example. We have a great game here in Australia - touch football. it is exactly what kids want in a ball game, as well as what parents will tolerate. Tackles have been replaced by touch only. As soon as a player of the opposing team is touched, he has to drop the ball so the opposing team player who touched him can take possession. They can run with the ball but while they have the ball all the opposing team will be chasing them. So it is in their interests to pass the ball to a member of their own team (and thereby change the focus of the attack). The only goals are a 'try' where the ball is manually placed over the goal line.
Very simple, loads of fun. Lots of physical exercise.
Maybe you can teach this to your son and his friend, and then involve the other boys. If your son already knows how to play this will give him status in their eyes. And because you will have to split them up to make two teams, some will be on your son's side in a friendly game.
You can turn around fear and bullying with friendliness, information and breaking down the walls of fear.
And if this doesn't work - well, at least you made an effort. That has to be of considerable credit.
Marg