I want to state a few points on this.
1) Learning someone to toughen up by hurting them, be it via headlock, punching, yelling at them, etc. is definitely not going to help them.
It is, in fact counterintuitive. It might make them 'shut down' even more and thus you would have accomplished nothing but the opposite of what you wanted.
2) Just because someone insulted your family or anyone close to you does not warrant violent behaviour. If you are mature as you say you are, you deal with it in a mature way.
What's a mature way?
Talking about it, discussing. Why did he say or do such things? Regardless what the reason is, you can simply tell him and state it's not ok for him to say or do those things and it hurts you and the person he's talking bad about a lot.
If he decides to continue such behaviour, you need to make clear to him you can't be friends any longer.
3) Taking the previous points into account:
Friends do not hurt each other, friends talk to each other about their issues, friends help each other.
I was actively going to a class to learn How to stand up for yourself. Yes, there are classes and courses about this for every type of person all over the world.
If you want to help him, find something like that for him. Tell him you think he needs to be more confident. That you're worried for him and want to help him.
With confidence, with learning how to communicate to others, how to be clear what you want, you'll be able to achieve much more than by learning how to physically hurt someone.
Of course, martial arts and self-defense are always good hobbies and could -in the rare cases you might be attacked- help.
In martial arts they teach you not to fight, but to only do actions that prevent harm to you, and deflect the attacker.
That all aside, a few other points:
4) A headlock can kill, even experienced people -reported as accidental- kill people with headlocks. It's been really recent a few people got killed due to a police officer or officers performing a headlock on people.
Not only in the US, but in Europe too.
Maybe it's fun and games between you and others, but it's still dangerous. As for your friend, I would avoid doing that altogether with him. He clearly does not enjoy it. I think he's not a wimp. He has been showing you clear signs he does not appreciate that, that he does not see this as friend-behaviour, that it makes him feel bad and horrible. And yet you continue.
Once again, if you want to be an actual friend to him, stop hurting him.
5) Whether or not someone has autism does not explain his 'wimpy' behaviour or any of such things. However, he does experience certain issues which 'normal people' may not. But then, his autism might make certain things easier for him, which might be difficult for 'normal people'.
Indeed, I agree with Razot that if you want to learn and understand him, you need to learn about autism. Really learn about it and don't go to 9Gag or some website where they joke about such things, all people who clearly don't understand the condition.
BUT everyone is different, autism does not make someone more 'bad' or more 'good' than someone else. Autistic people learn things differently. They're not aliens, they are humans with brains that function differently.
If properly communicated, they can function just like anyone else. But doing the opposite will make them shut down to those around them.
Those suicidal thoughts you mentioned he has? You hurting him? For him, it might be connected. He might feel no one is really helping him, that he has no true friend.
So all in all: Don't hurt him, talk to him. Try to understand him and actually help him by pointing him in the right direction.