Credit: Kevin Mazur/WireImage

Justin Bieber and Selena Gomez aren't letting that pesky paternity scandal come between them.

A source tells Us Weekly the 17-year-old singer "was in great spirits when he came for a couple hours to rehearse" for the American Music Awards on Saturday.

In the front row? The singer's 19-year-old girlfriend, who stood by him when Mariah Yeater first accused him of fathering a baby boy.

"At one point Justin even sang 'I Feel Good' a la James Brown!" the source adds. "They were totally adorable. They were snuggling in between takes, and he was serenading her! They look as in love as ever."

Bieber -- who still plans to take a DNA test despite Yeater dropping her lawsuit -- is scheduled to perform a song from his new holiday CD, Under the Mistletoe. Gomez is slated to present at the awards show, which will feature performances by Jennifer Lopez and Nicki Minaj.

The American Music Awards airs live on ABC Sunday at 8 p.m. EST.