What is he going to do tomorrow?
be going to是一个将来时的固定问法.
补充:
I'm going to visit BeiJing tomorrow.
表示一般将来时,句中be going to是一个将来时的固定搭配,凡是有be going to都一定是表示将来时
I'm visiting BeiJing tomorrow.
有些动词,如：arrive,be close,come,do,done,go,have,leave,open,play,return,sleep,start,stay等,用于一般进行时,并且通常与一个表示将来时间的时间状语连用,可以表示将来时.