Why isn’t Jess wearing a shirt or shorts?

Jess goes running in overalls because he thinks he'll be too hot if he wears clothes underneath them.
Jess gets up early every morning in the summer to go running; he's up early enough that he worries his mother will get mad if he wakes her. He wants to improve his speed so that by the time school starts, he's the best runner in the fifth grade.
When he wakes, he puts on his overalls right away. He doesn't bother with a shirt. Paterson writes:

He didn't worry about a shirt because once he began running he would be hot as popping grease even if the morning air was chill, or shoes because the bottoms of his feet were by now as tough as his worn-out sneakers.

The exertion of running will heat up his body so that he won't feel the cold air on it. When he leaves the house, Jess notices that he can see his breath in the air even though it's August. He thinks, too, that by the time he's doing chores in the afternoon it will be plenty hot. By the time he gets home, he's aching and sweaty.