Haley's 21st Birthday - S6-E10
Plot hole: The family mistakenly believes Hayley will be having her first drink. However, there were several instances in earlier episodes where they saw her drinking or drunk, or she admitted she'd been drinking. For example, in the episode "Arrested", the family drove down to bail her out of jail for underage drinking, and they saw the video footage, and the family members who didn't go were informed of the situation.
Answer: He has seen himself dancing, and is mortified at how bad he looks.