Dennis had to have met Shawn earlier than that. At the July 5th, 1981 show in Long Beach, Dennis shouted out "I love Shawn!" during "Help Me Rhonda". On a related note, I'm surprised that Mike didn't beat the living merda out of Dennis on that occasion.
Hmmm...I've always thought he shouted out "I love this song!"...but maybe not.
Watch the dvd, or listen to the recording again. About 10 seconds after Dennis made the comment, he also said "I love you Shawnie".