前面一句的is就是指“what he enjoys” 即“他喜欢的东西”，这里是把“what he enjoys”当成一个整体了 所以用is。being 一般用在进行时中，而这句里没有说时态的事。
What he enjoys 名词性从句做主语is系动词 to play basketball with his friends. 表语
what he enjoys是主语从句
