Old man Glassings is Partridge's one-time world history teacher, and he's pretty much the closest thing Partridge has to a father.
We'll give you a refresher course on Glassings: in Pure, Glassings plays a limited role because Partridge is outside the Dome most of the book. All we know is that Glassings is a good guy.
However, when Partridge comes back inside in the middle of Fuse, Glassings fills him in on the evil plans of Ellery. Plus, when Partridge has his memory erased, Glassings sneakily throws in a "welcome back" comment that jogs Partridge's memory— "welcome back" isn't normally something you'd say to someone who was supposedly in a coma.