Divatilya, he did sing it. It was just while he was still with the Commodores. I can't imagine any voice other than his singing that song and if I'm not mistaken, he also wrote it.
It should be BoyZ II Men, with a Z. I've never liked intentional misspellings, but that one doesn't bother me for some reason. Maybe because they were my first concert. :-)
I think you should accept alternative spellings for Boyz ll Men. I am typing it with a double lower case l because I literally don't even know how else to get that 2 symbol. I tried "Boyz 2 Men, Boyz to Men, Boys to Men, Boyz 11 Men, Boyz ll Men, all different variations with z's and s's and still missed the question...Like come on, seriously?!?!