On some level I wouldn't force it. If your child is high functioning to the extent that it's difficult for the outside world to understand that he has a medical condition: Then he's probably not more annoying than lots of little brothers to be honest. My kids can drive each other absolutely INSANE. Sometimes I empathize. Sometimes I tell them not to tattle and to work it out themselves. Sometimes I discipline them. Sometimes I separate them....
There's a level of cruelty that IS NOT ALLOWED no matter how annoying a sibling is being. And there are rules to be followed for all. Lots of times if they don't want me to get anyone in trouble they'll resolve their own fights.
I assume your Autistic child has rules like ay other child. If he's high functioning then he knows what "Do not mimic" means. It may be even harder for him to resist the urge to mimic than other kids, but he can. And he can also understand that if he does mimic, his brother will get extremely annoyed, push him, scream at him, not want to play with him, etc. It would not serve the brothers in ANY way, to allow that behavior in the younger one and only try to deal with the older one's feelings about it. This would build resentment. When my kids mimic each other they drive each other to tears and violence, but they don't even tattle anymore because they know I'll discipline the mimic-er. Mimicing is ANNOYING and all bets are off as to what will happen to you if you do it. And even healthy kids LOVE to mimic. Same with other annoying attention-getting behavior that kids in the 2-5 yo age range LOVE to do to provoke reactions. You do it, SOMEONE'S gonna get mad. A sibling or playground kid may punch you and a parent may discipline you, so don't do it. Or be prepared for the reactions of others if you do.
So I get it that you have challenges that other people don't and behavior that is even harder than for most to control in your youngest. But the best thing you can do for your older son is to model patience and love for your youngest ALONG WITH discipline so it doesn't feel like the youngest is always getting special treatment. If your oldest sees you trying your best to enforce rules, he will not resent being unfairly treated. If you show lots of love to oldest and appreciate his help, etc, he'll learn he's appreciated.
But I wouldn't teach "Your little bother can't help himself and you need to be empathetic" too too much if that makes sense. This will short change both of them in the real world.