John, 63, collaborated with Morrison, best known for playing Mr. Schuester on the US show, on a track for Morrison's debut album.
Asked about the duet, John said to Parade.com: 'I just did it! It's a duet. It's great, really fantastic. He sings great on it.'
He added: 'It's a medley of Mona Lisas and Madhatters going into Rocket Man. I did it on Monday.'
The new dad added that he believes British rapper Plan B is the next best thing.
He said: 'There is Plan B, which is a British act out. It's going to [be] released here [in America] in March on Atlantic. It's a huge record in England.
'It's a bit like a male Amy Winehouse record with a little bit more edge. He's a 27-year-old white boy who has the voice of Smokey Robinson.'