Dennis relaxed a little into the chair when Sebastian said he would speak with Professor McGonagall. It hadn't worked well in the past, but maybe another teacher talking to her would be better than a student approaching her on his behalf. Because that was clearly what had happened.
When Sebastian told him to stay and walked away from the table, Dennis' first thought was to run again. But he fought against the desire and stayed in his seat. His hands were wrapped tight around the arm of the chair though, showing the struggle to not run. His knuckles were white with the force. But he did it. When Sebastian returned with the most ice cream EVER Dennis' eyes widened. Everything was made better by ice cream in his book.