Tom is Doug's ten-year-old brother, who can always be counted on for a reality check. There's a big difference between how you see things at age ten, when you're still safely a child for another year or two, versus at age twelve, when you're on the cusp of adolescence and terrified by the responsibilities of growing up. When Doug bemoans God's failure to run the world to his satisfaction, Tom tries to cheer him up, saying, "He's all right, Doug. He tries" (22.11). Thanks for the reality check, Tom.
As readers, we need Tom to help pull us back from the brink of Doug's emotions, which are pretty overwhelming at times. If Doug's intent on having a philosophical and spiritual awakening, Tom's quite happy to do the opposite—he's perfectly content taking life day by day. When Doug asks Tom whatever happened to happy endings, Tom has a simple answer:
"All I know is I feel good going to bed nights, Doug. That's a happy ending once a day. Next morning I'm up and maybe things go bad. But all I got to do is remember that I'm going to bed that night and just lying there a while makes everything okay." (29.7)
Whoa—we know Tom's only ten and all, but he seems pretty wise, doesn't he? God's trying and going to bed's a happy ending every day. Looks like somebody does a pretty solid job of keeping things in perspective.
Tom has his own unique way of seeing the world, what with his theory that adults never were children (16.17)—they're just that different from kids. Tom knows when the adults have stuff to do and figures he might as well step out of the way and let them do it, not troubling himself with the problems of a group of people he and Doug see as "a different race" (6.16). He's a true kid, content that adults are capable of making things work out for the best and that their world just isn't his.
In a book in which we see so much of the adult world, Tom represents a safe return to the land of childhood whenever Doug is in his presence. When Doug grapples with the tough questions of life in his notebook at night, Tom's the one who's by his side, in the other bed, waiting for him to turn out the light and go to sleep. He's Doug's anchor to childhood, as well as his biggest fan.
When compared to Doug, Tom can appear to be somewhat lacking in insight, but he's also a stark reminder that sometimes we make things too complicated. He's simultaneously imaginative—he's totally able to go on Doug's flights of fancy with him—and grounded, seeing Doug's troubles not as a dark night of the soul, but rather a series of unfortunate childhood mishaps.
We see this perhaps most starkly during Doug's illness in Chapter 37. When Mr. Jonas comes by and sees Tom crying because he's afraid Doug will die, Tom explains Doug's troubles like this:
"It's been a tough summer. […] Lots of things have happened to Doug. […] [H]e lost his best aggie for one, a real beaut. And on top of that somebody stole his catcher's mitt; it cost a dollar ninety-five. […] And worst of all, I grew one inch taller, catching up with him almost." (37.70-74)
Doug may have worried himself sick, but Tom doesn't get the whole existential dilemma thing; his brain's still firmly lodged in child world. From where he's sitting, his brother's been outgrown, lost valuable toys, and other childish problem. But his concern for his brother touches Mr. Jonas's heart, and Mr. Jonas invents the "potion" that saves Doug's life—which you might say is a childish solution to a very adult problem. And it not only works, but only happens thanks to Tom. Childhood for the win.