Backstreet Boy A.J. McLean has the ultimate New Year’s Eve surprise for his live-in love Sarah Martin — he plans to pop the question!

And despite his bad-boy rep, A.J. asked for Sarah’s hand the old-fashioned way — he called her mom!

“A.J. and Sarah have been living together for a few months,” an insider tells The ENQUIRER.

“But he called Sarah’s mom and asked for permission to marry her daughter.

“Sarah’s mom almost dropped the phone in shock, but she quickly gave her consent.

“A.J. made the call in early December, so Sarah’s mom has been struggling to stay mum ever since.

“Something this big is hard to sit on. But she vowed to keep the secret.”

A.J., 23, recently did a stint in rehab. He met Sarah, a 26-year-old singer, at a karaoke bar before he got sober. But she’s played a key role in A.J.’s recovery.

“They moved into a home in Hollywood together and bought a puppy they called Serenity,” said the insider.

As for his new love and his sobriety, A.J. says: “I thank God every day. Life is good.”