La Boheme may have a melancholic theme, but I don't get an overly sad, gentle mood from it, it's pretty passionate actually.
The singing is quite restless in several parts of the song, and the last verse is sung in a lamenting and fervent manner. Add to that the way, that the song speeds up right in the end, then it's kinda hard not to look grim and fierce during the ending pose.
I guess Nathan could put on a less 'angry' and more desperate expression. Which is something that he actually does throughout the song, when you watch his face,
so I'm sure it's just the increase in tempo that riles him up in the end.
ETA: Plus, it's still a very new program for him, and with all those ice shows that he's done these past few months, he probably didn't have that much time to practice and familiarise himself with it.
So maybe he just fell back on that type of ending pose + expression out of habit, and
it may get adjusted and fine-tune later on (or not *shrugs).