Terry loved his family. There was no doubt about that, and he took his responsibility of second eldest seriously. However, there were days that the grief threatened to overwhelm everyone in the house, and tempers ran short. This was one of those days. After the third time one of the twins walked in without knocking and messed up his books, he knew he had to get out before he said something cruel. So Stephen's journal message came at the perfect time.
By the time the House Elf (and he had to grin a little at that; no one ever said Stephen wasn't generous with his family benefits) he was beyond ready for a day of mindless fun with his friends. He'd let his mother know he was going out, mostly because despite his antipathy towards his family at the moment he didn't want to unnecessarily worry her, and let the House Elf Apparate him away.
He popped back into existence, for lack of a better word, and saw that Stephen had, indeed, set everything up. He grinned broadly, let the tension fall away, and went to meet up with his friends just in time to hear Anthony tease Stephen about running away.
"Our esteemed Stephen would never run away. He's merely abdicating responsibility for the day."