He is on the wind up. 點解?

Wind up can be an adjective or noun.

In your sentence, it is a noun which means that somebody says or does something in order to be deliberately annoying, especially as a joke.

If it is an adjective form, the meaning of "wind-up" is that you intend to bring something to an end.

Hope the above helpful.

'wind up' 可解作 '結束', 無上文下理, 估計佢講 '他正在結束他的生意'。