I recently saw a documentary on Simon and Garfunkel filmed around the time of Bridge Over Troubled Water. There was a guy in it who was Paul Simon's spitting image - came to find out it was his brother Eddie (who knew??). Anyway, a search on Eddie brought this to my attention - he sounds as much like his brother as he looks too! Anyway, this sounds like it could have been written about Dennis!
http://youtu.be/XA3Z-vtBI1U