Backstreet Boys' A.J. McLean Jammin' Out to BSB

10/24/2015 12:10 AM PDT

Backstreet Boys' A.J. McLean Jammin' Out To BSB Cover Song

EXCLUSIVE

A.J. McLean got to relive his Backstreet Boys glory days in front of a hard-partying crowd ... all thanks to a '90s cover band.

McLean hit up a '90s Throwback Party at Greystone Manor in West Hollywood Thursday night and when a '90s cover band started playing the BSB hit "Everybody," McLean went nuts.

Check out the video ... A.J. kills it on air guitar.

Backstreet's back ... sorta.