I've uncovered something huge into finding out who the man that inspired
21 was. Adele dated him during the 'An Evening With Adele' tour, and apparently Adele did an interview and refused to give a lot of details but revealed her boyfriend at the time (which was right in the middle of the tour) was named Michael, according to
this article. It was rumored to be Mika, but they're just friends and regardless, they're only about 4 years apart. Adele stated the age difference between them was more like, 10 years apart, if I remember correctly. And it was
confirmed that Mika and Adele never dated. So, we don't know who it is, but we know his first name!