The crowd at the Plano Senior High School graduation ceremony could not believe their eyes when they witnessed a student with autism take the microphone on stage and deliver the graduation speech.

Much to the surprise of his classmates, Sef Scott, a graduating senior with autism — a boy who rarely spoke in the class — delivered a six-minute-long commencement address.

"I would imagine that to the seniors that know who I am that it is entirely unexpected that I would be standing here giving a speech," Scott said. "I have autism and a social communication disorder. While I have the ability — while I have a vocabulary that you do and I have the ability to physically produce spoken words, it is not the normal thing for me to electively speak. Just by my being here, speaking to all of you — that alone is unexpected.”

So where did he find the courage to take such a big step? Scott said that he was inspired by his brother, Sim, who survived a brain tumor. His brother had given speeches reiterating his experience and that pushed him to speak up.

"My brother Sim is a 9-year brain tumor survivor who has given dozens of speeches to raise money for various children's charities since 2009. So with his help and that of my mother, we worked sentence by sentence to make my voice heard," Scott explained.

In the powerful speech, Scott spoke about the importance of living life to the fullest, even without knowing what lies ahead.

"Don't follow someone else's dreams. Don't waste time on something you never wanted. Do the unexpected. It is your life that you are living, not anyone else's. So do what fulfills you. Don't fear the future. Don't fear the unknown. Will it be unexpected? Yes. Yes, it will. But that does not make it wrong," Scott told the audience.

“Be the unexpected like me. Do the unexpected for the benefit of others. Live the unexpected for your own happiness,” he added.

Vicki Scott, Sef’s mother, told ABC-affiliated WFAA: “He knows exactly what’s going on around him, but he doesn’t engage. Something about his spectrum doesn’t allow him to have the back and forth like you and I are.”

That was why she was taken by surprise when Sef agreed to audition for delivering the graduation speech.

"Our goal was to have his voice heard in whatever way he could manage and so many people have taken away so much more than I ever expected," said Vicki. "It affected me so profoundly. I could hear all around me, other people were crying, too."

He received a big applause and standing ovation for his speech. A video of the speech, which was uploaded on Facebook by the school district and has been viewed almost 500,000 times and shared over 7,000 times.