The 45-year-old writer has previously revealed the Nashville-based band refused to allow their hit single 'Use Somebody' to be featured in the musical TV show, and Ryan has now claimed the band are "missing the big picture" by not opening up their back-catalogue for use.
He said: "F**k you Kings of Leon. They're self-centred a*sholes, and they're missing the big picture. They missed that a seven-year-old kid can see someone close to their age singing a Kings of Leon song, which will maybe make them want to join a glee club or pick up a musical instrument.
"It's like, OK, hate on arts education. You can make fun of 'Glee' all you want, but at its heard, what we really do is turn kids on to music."
However, King's of Leon frontman Caleb Followill has hit back at Ryan, claiming the vitriol has "gotten out of hand".
He told Hollywood Reporter magazine: "This whole 'Glee' thing is a shock to us. It's gotten out of hand. At the time of the request, we hadn't even seen the show. It came at the end of that record cycle, and we were over promoting 'Use Somebody'.
"This was never meant as a slap in the face to 'Glee' or to music education or to fans of the show. We're not sure where the anger is coming from."