Steve - it was line/refrain/line/(pause for reaction), with a nonsense refrain that sounded... well, folky. I can't for the life of me remember what syllables he was actually using - I think it was something fairly simple, though, like 'hi-diddle, hi-diddle, hi-diddle-ol-day'. And don't ask me to guess at the tune - he wasn't a great singer.

All in all it went something like this:

Boastful comment about his penis
Hi-diddle, hi-diddle, hi-diddle-ol-day!
Disparaging comment about his friend's penis (in rhyme)
(pause for audience reaction)

And repeat, with (slight) variation of subject matter.

He looked about 20, as did his friends. We do get people of that age group at the local Folk Club, but it's a "folk began with Dylan" kind of club, so the chances of him being exposed to songs with nonsense refrains that way are pretty slim. Also, it's rare to hear anything seriously bawdy these days, at the FCs & singarounds I go to at least - probably because they're not men-only environments. All quite odd.