如果你想用英文表达"他不想变胖"，可以说"He doesn't want to gain weight"或者"He wants to avoid gaining weight"。

"Gain"在这里指的是"增加"，而"weight"指的是"体重"。所以"gain weight"就是"增加体重"的意思。

例句：

He doesn't want to gain weight so he's been exercising and eating healthy. 他不想变胖，所以他一直在锻炼和健康饮食。

She wants to avoid gaining weight during her pregnancy. 她希望在怀孕期间不会增加体重。

翻译为中文：

他不想增加体重，所以他一直在锻炼和健康饮食。

她希望在怀孕期间不会增加体重。

希望这些信息对你有帮助。