Backstreet Boys singer Brian Littrell could barely keep it together as he stood outside the doors during his 22-year-old son Baylee's audition for "American Idol" on Sunday night's show.

Brian got visibly emotional as he told cameras how "proud" he is of his son during the Season 23 premiere of the singing competition, explaining ... "He's obviously grown up in the business and I think he's cut out for it."

The proud papa was also thrilled to join his son in his audition at judge Luke Bryan's request. They seamlessly harmonized to the chorus of Baylee's original song "Waiting on Myself to Die" ... and Brian choked up again as Baylee was praised by all three judges -- including Lionel Richie and Carrie Underwood.

He exclaimed, "I'm crying!" before Baylee seemed to tear up as he thanked the judges.

Country crooner Carrie told Baylee he can go far in the comp ... so fans will have to sit back, watch and -- of course -- vote when it comes time for that phase of the show.

Baylee sang his father's praises during the audition process, also ... saying he thinks Brian's career "is one of the most extraordinary in music history."

He also recalled opening up for the Backstreet Boys as a kid, saying ..."I just had my two little songs and I came off stage and when the guys went on, the screams and the lights went down. I was just mesmerized."

Baylee also opened up for the '90s boy band when he was a teen during the North American leg of their "DNA" world tour ... so it seems like he's had plenty of practice to hone his craft before taking this next big step.