Law enforcement sources tell TMZ ... Tommy's neighbor found him dead in his Las Vegas home around 8 PM on Tuesday. No foul play is suspected.

Tommy was a teenager when he got his big break in "Old Yeller" ... the story about a boy and his heroic dog. He was Disney's go-to kid actor to fill that All-American persona ... leading to roles in other Disney films, including "The Shaggy Dog, "Swiss Family Robinson" and "The Misadventures of Merlin Jones."

He was discovered in 1954 as a 13-year-old while performing opposite Will Rogers Jr. for the play "Ah, Wilderness!" at the Pasadena Playhouse. Legend has it, Tommy had auditioned for a role nobody else bothered to audition for, so he got it. He had only 5 lines in the play but it was enough for a talent agent to sign him and cast him in an episode of "TV Reader's Digest."

Tommy ultimately ended up getting cast in "The Mickey Mouse Club" ... and appeared in several other TV shows. But, there's no doubt his role in "Old Yeller" shot him to stardom.

But, his time with Disney didn't come without controversy. According to some reports ... Walt Disney personally fired Tommy in 1964 because the 21-year-old was gay.

Tommy once said, "When I was about 17 or 18 years old, I finally admitted to myself that I wasn’t going to change. I didn’t know what the consequences would be, but I had the definite feeling that it was going to wreck my Disney career and maybe my whole acting career. It was all going to come to an end."

He added, "Eventually, I became involved with somebody and I was fired." Tommy was still cast in films after that, but he admitted they were some of the worst movies he ever made.

Despite the Disney fallout ... Tommy was inducted as a Disney Legend in 2006.

He was 79.