Ray stops running because he realizes the consequences of not making the football team and disappointing Coach Brennan.
The two sentences in the passage that explain what causes Ray to stop running are:
- Ray bit his lip. Harry was right, of course. The two had promised each other that they would both make the eighth-grade football team.
- Several angry parents of would-be football players had called the school and asked for Coach Brennan to be removed.
In the first sentence, Ray stops running because he realizes the consequences of not making the football team and disappointing Coach Brennan. In the second sentence, the information about the angry parents calling the school suggests that Coach Brennan's demanding physical training in the heat may have been a contributing factor for Ray's caution.