© Courtesy Sarah Tassi Sarah (left) and Preston Tassi

Sarah Tassi waited a long time to see her son, Preston Tassi, smile. Five years to be exact.

At just 3 days old, Preston was diagnosed with Moebius syndrome, a type of congenital facial paralysis that results in underdeveloped facial nerves. The condition left the boy unable to smile, eat properly, and also resulted in vision and speech issues. He was also born with a deformed right hand.

“There was sadness, of course,” Sarah tells PEOPLE of the moments she and her husband, Jason, learned of the diagnosis. “When you’re a new mom, you want to see your baby smile.”

“Drinking from a bottle was difficult for him because he couldn’t make his mouth come to the bottle. So we figured out a way to hold the bottle with our hands while pushing his cheeks in a little bit to help him eat. You just do what you gotta do and make it work.”

© Provided by TIME Inc. Preston (left) and Sarah Tassi

As he grew, Preston learned to push up the corners of his mouth with his hands to “smile”, but was never able to do so on his own — until this year.

In February, Preston underwent the final end of a two-part facial animation surgery in which surgeons at St. Louis Children’s Hospital removed nerve muscles from his inner thigh and inserted them into his cheeks.

“It’s scary having your little one under anesthesia for 10 hours or more,” Sarah says, noting that Preston underwent the first portion of the surgery last August. “There were a lot of emotions.”

© Provided by TIME Inc. Preston Tassi

It can take several months for the surgery to produce a smile, hospital staff said in a statement to PEOPLE. But just three months after the final surgery (two weeks before his 6th birthday), Preston was able to sport a wide grin. Sarah says she’ll never forget seeing her son smile for the first time.

“It was Mother’s Day, we were in Kansas City with his cousins. There was a birthday party and somebody was like, ‘Preston, let me take a picture of ya.’ And he turned around and he grinned at them,” Sarah recalls. “It was just amazing. It’s the little things you take for granted. I broke down and cried.”

© Provided by TIME Inc. Preston Tassi

Sarah works to raise awareness of the rare condition and the doting mom says she’s happy to share her son’s story to get the word out about the syndrome, which impacts 2 to 20 per 1 million people, according to the Moebius Syndrome Foundation.

As for Preston, he has a new lease on life and just can’t keep from smiling.

“It’s breathtaking, There’s really no words. It’s simply amazing,” Sarah tells PEOPLE. “It’s something you want to see your child be able to do and for so long not being able to do that it was sad. But at the same time I knew he was happy. He was still a good, happy kid and it’s just nice to be able to see it on his face now.”

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