As usual, since the victim was African-American and the assailant white, this automatically becomes a race related issue. Had the victim been white, would the assailant reacted in the same way, seems to be the question. Quite possibly he would have. Autistic children can seem somewhat aggressive in their mannerisms, and not knowing the teens condition, the assailant misconstrued the teens intentions.
I am a huge supporter of probation and community service in lieu of prison time. Perhaps they could have given him more hours of community service, and perhaps a fine, although I suspect there is one and it just wasn't mentioned. We need to be practical when we punish people. We don't need to support people to rot in the prison system. Forcing community service provides free work while at the same time punishing the person.
The sentencing in this case seems to be in line with similar cases. As long as defendants are treated equally under the law, then we have some assurance that the penalties are appropriate. Say the wrong thing during an argument, and shoving can turn into assault for anyone. Don't you want to be treated fairly in court. However, because the defendant has a history of aggressive behavior toward teenagers, he bears watching.
Ultimately, the judge that knew the case knew all of the circumstances and made the right decision. Three years of probation isn't easy to do, and the person has to behave well if they want to stay out of jail. It's less expensive for the system to have a peron on probation, rather than in jail.