Why did Stanley decide to lie to his parents about what he is doing at camp?

In his letter to his mom, Stanley lies about what life is like at Camp Green Lake. He doesn't tell her how hard it is; how exhausting and back-breaking it is to spend your whole day out in the hot, baking sun; how he's so physically wrecked at the end of a day's work that he can barely stand up in the showers. Instead, in his letter home, Stanley just says that he's "pretty tired." He also says that he's been making friends and that he'll get to learn how to water-ski once he's passed his swimming test, his parents not knowing that Green Lake is dried up—no longer a lake at all.
Stanley won't level with his mom because he doesn't want her to worry. If he told her the full story then she'd spend every minute of the day fretting about her son, and Stanley doesn't want that. We can infer from this that Stanley has a good relationship with his mom, which is more than most of the boys at Camp Green Lake can say.