1) Your undying love and support. They do not need to mistakenly think "oh, I was normal until the vaccines. And then I started causing mommy trouble." As an autistic child, he is more likely to make this assumption through the misinterpretation of your intent.
2) He needs to be taught to take responsibility for his actions. He needs to realize that autistic or not, if he does something inappropriate or wrong, he still has to pay the consequences. He can't always blame his issues on other people or society. And what do you think telling him "your doctor made you who you are," is going to teach him? To blame others and never take responsibility.
3) He needs a strong support network. But if you've a) made him think he's causing you trouble ("I was normal and easy until that DAY that I became different and difficult") and b) to blame society for his actions and issues and not himself, he will never learn to cultivate the trust he needs to build the support networks he will need as he grows older.