Some people have to be dealt with on their own level. They don't understand anything above that. So you can either deal with them on terms they can understand or put up with their being obnoxious. It is a fact of life. Think about it, dear heart.
I wish I knew his first name so I could figure out if Pants-less would go well with it. For instance, Pants-less Pete would be a good name. You also might want to share this little ditty from American Idol with him.
'Y-man' (as in 'Y'-Fronts) might be a start. Just plain 'Cheeks' as a follow-up, followed by his surname (as in 'Bugsy' Segal), or on its own. In Geordie-land (Newcastle-on-Tyne) it would be 'Y-I' man, (as in 'Why aye', meaning 'of course').