"Why is he late?"
Translation:Warum ist er spät?
To elaborate on that, including little differences between persons and things:
Er verspätet sich. - He/it is being late, he/it will come later (person, train)
Er kommt verspätet. - He/it arrives late (person, train, letter)
Er ist verspätet. - (He)/it is late; not really used for persons, but mostly for e.g. trains.
("It is late" in the sense of "It's already 11 p.m." = "Es ist spät")
Er ist spät dran. - He's running late (he has to hurry to finish / be there in time); normally only for persons
Er ist zu spät. - He/it is too late - sounds a little bit non-standard, like that sort of anglicism you get by "translating" English phrases straight into German. But "Es ist zu spät" is totally fine for "It's too late".
Er kommt zu spät. - He comes too late. The more elegant way to say "Er ist zu spät". (person, train, letter)
(Er kommt spät. - He comes late (e.g. late in the day, or shortly before a deadline. Not "too late"!)