So he's got a restraining order. He's autistic but seems to know where whoever he is harassing is online. That suggests he knows enough to know he's trying to make contact. Yet he understands a restraining order enough to understand he shouldn't be making contact.
This doesn't add up as him lacking in understanding. He understands what he's doing. That's why he's doing it.
He wants to continue to harass someone and is looking for multiple ways to do it - and thought he could get away with it being it was online.
The fact he's had his computer taken before for examination suggests that he's deliberately using the internet to bother people. He also understands enough that he didn't want his computer taken away for another 18 months.
This begs a question. At 19 and being on this high level of disability, I presume he's still living at home. Given you believe he's not not capacity to fully understand what's going on and the internet, and he's got form, why do you allow a vulnerable autistic person unfetted access to the internet?
What's clearly being demonstrated here is that you, as the adult who presumably has caring responsibilities on some level, isn't keeping a firm eye on what he's doing and instead are trying to push responsibility on to others for not making sure he has an appropriate adult
You can't have it both ways on this OP. Either he shouldnt have free access to the internet - which comes back to you - because he can't be trusted on it and he extremely vulnerable and doesn't understand what he's doing. Or you agree he has enough capacity and understanding to use the internet freely, in which case he fully understands what he is doing and is responsible enough for his actions and doesn't need a responsible adult in an interview.
You need to wake up and decide which it is, because this is going to be a long term issue with consequences. You can't infantilise him when it suits you at the expense of others or you need to face up to the fact that he's knowingly causing harm and distress to others.
What happens now, because one or both of you haven't taken responsibility, is a mess. One which needs proper legal advice. But you really need to take a long hard think about this and your role / relationship with your son and his access to computers.
It is not typical to have your computer seized for evidence. That says something really bad is going on. Perhaps you need to do some reflection on the impact on the victim in this too because it sounds like the restraining order issue has been going on for well over 18 months dating back to when he WAS still a minor who was definitely under your responsibility. You need specialist help and support but you also need to remove computers from the equation to limit opportunity for this to repeat.
I'm sorry for this reason, I do lack a certain amount of sympathy for you based on what you've said. The internet clearly isn't a suitable place for your son either because he's too vulnerable to go on it unsupervised or he's a risk to others. There isn't a middle ground here unfortunately.