Aidan Duncan is happiest when he’s playing sports on a field or court. But the 12-year-old boy from Indianapolis, Indiana, is different from the other children on his teams. He was born with a partially formed left arm.

“We found out that his left arm was not developing. It could have been because of his umbilical cord wrapping around his arm, they’re not quite sure,” his mother, Mona Mulvany, told “Good Morning America.” “They never did have an exact answer as to why this happened.

A specialist told Aidan’s parents that their son would have balance issues and that running would be difficult for him, but Aidan has defied those predictions by dominating the field in in football, soccer — his favorite sport, basketball and baseball.

“For the All-Star team, I’m probably tied for first with the house league team, I’m probably the fastest player on the team,” Aidan said, adding that he also likes to swim “for fun.”

While he acknowledged that it “takes probably more effort,” Aidan, who has been playing sports since he was about 5 years old, said it “hasn’t been as rough as people think.”

“It takes a lot of hard work and effort. And you can’t really give up. You’ve got to keep your head up and not give up,” he said, echoing a lesson taught to him by his parents.

Courtesy The Duncan Family

His parents have also “really kind of ignored the fact that I have one arm. They just treat me the same, as anyone else … it made it easier for me, made me feel more normal and stuff,” he said, adding that he hasn’t been bullied by his peers.

“A lot of kids are curious but never bullied me,” he said.

Like every 12-year-old, Aidan has his own sports hero: Matt Carpenter, the third baseman with the St. Louis Cardinals baseball team.

When Aidan’s story made headlines in the Indianapolis Star, the boy got a surprise from Carpenter in the form of a video message in which Carpenter congratulated Aidan on his skills and invited him to a Cardinals game.

Aidan was at the game this weekend, and got to meet Carpenter in person and have a bat signed by the pro player.

Aidan’s parents say they don’t treat him any differently from his two siblings.

“Aidan doesn’t complain and he just goes through life – this isn’t an issue for him at all. And I want other kids to know that,” Mulvany said. “And I want other people to know that when they see someone like Aidan, don’t think of disabled. Don’t think handicapped. He is incredibly able and can do anything that others can do.”

Joe Duncan is an assistant coach on his son’s baseball team, the First Baptist Athletics All-Star team.

“We refer to his arm as his little arm,” Duncan added. “And it becomes a little bit more challenging for him. But for the most part, it just takes him a little bit longer to tie his shoes. But he can do that and do it well.”

Aidan has some advice for other children who may face challenges that are similar to his own.

“I would probably say that you can just — got to go through, never like ask why it happened or why you’re like this, you’ve just got to ignore that fact and keep your head up and if something goes wrong, just keep looking forward,” he said. “Look to the future and never look back.”