"Dude, you have so many people that love you," Strong says in the video. The actor said that he received call and texts from all over the world alerting him to Alex's story, including from other Boy Meets World alumni.
The #Rider4Alex campaign began when Alex's brother reached out to the Boy Meets World actor on Twitter.
Alex, who turned 13 today, is a die-hard Boy Meets World fan, and his family knew nothing would make his special day extraordinary quite like a message from Strong, his favorite star.
Jackson Hengsterman told ABC just how much Alex loves Strong and his character Shawn Hunter.
"He loves the move Shawn Hunter does in the show where he spikes his hair up, like in the video I posted," Jackson said. "He does the hair-spiking often, especially when he sees some 'ladies' he is trying to impress. It's hilarious."
In the birthday message, Strong offered Alex some tips on perfecting the Shawn Hunter hair flip.
"You want to get it to two hands, 'cause that's when the real Shawn Hunter magic happens," Strong said, demonstrating his technique.
Alex's mom, Stacey Hengsterman, also told ABC just how much a birthday message from Rider would mean to Alex.
"Just the thought of Rider saying that he heard Alex was his biggest fan and he was thinking of him would be something Alex would talk about for the rest of his life," Stacey said.