"He is not smart" 或者 "He is not intelpgent" 都可以表示他不聪明。
来源: 这是一个常用的英语表达，不难理解。
例句:
"He may be good at sports, but he is not smart." (他可能擅长体育，但是他不聪明。) "I don't want to work with him, he is not intelpgent." (我不想和他一起工作，他不聪明。) "He is not very bright" (他不太聪明)