EVANSVILLE, Ind. — On the evening of Dec. 9, a friend of Becka Anderson’s was driving when he saw a man walking toward the airport in Evansville, Indiana. .

It looked a lot like William “Bill” Anderson, Becka’s 22-year-old son.

It would have been quite a hike, but long strolls were nothing out of the ordinary for him.

But that evening, something was wrong. Bill is on the autism spectrum and lives with Tourette’s, which can cause him to lapse into flailing arm movements and uncontrolled vocal tics. Medication usually controls them, but he’d recently decided to stop taking his meds.

Things were OK for a while, Becka said. Eventually, though, his mood darkened.

He started holing up in his room, refusing to even eat with the family. He only came out when he thought everyone was gone or asleep. On Dec. 9, he stepped out of the house and never came back. His mother reported him missing the next morning.

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That’s when her friend told her about what he saw. If that really was Bill, that was the last time anyone connected to the family saw him in Indiana.

They’ve since learned he flew to New York City, landing at John F. Kennedy International Airport around 8 a.m. Dec. 10. His bank card was subsequently used near Long Island. But they don’t know where he is — or who he might be with.

Bill’s disappearance is the culmination of a tough stretch for him, Becka said. He thought his family was trying to control him when he received a court-appointed guardian, and Becka believes he’d been wiring money to someone he met online.

He was also buying slews of gift cards for things he never used, such as Google Play.

“We just want to find him, bring him back to Indiana, and get him the help and medical attention that he needs,” she said. “He’s a really funny, kind man when he’s properly medicated.”

Bill stands about  6 feet tall and weighs 150 pounds. He has brown hair and brown eyes and was last seen wearing khaki cargo pants, a brown long-sleeved shirt and boots – although he could have easily changed in the days since he left home.

The family asks anyone who might see Bill to avoid approaching him and call police instead. A flyer posted to Facebook by the AWARE Foundation – a national nonprofit that raises awareness about missing persons – warns that Bill might be a danger to himself or others.

The family discovered he’d flown to New York when his guardian went to the Evansville airport with court documents in hand, Becka said.

But narrowing the search down to a giant city like New York isn’t really narrowing it at all. The receipts from his bank card are frustratingly vague, too. One just reads “TAXI.”

Before his disappearance, Bill was a beloved member of the community. Last month, a local news station named him their “Hometown Hero” for the week of Thanksgiving thanks to his work with Goodwill.

“I only go up from here,” Anderson told the station at the time. “The sky’s the limit. And I’m going to break that limit.”

Follow Jon Webb on Twitter: @JonAdamWebb