Omovo is most likely irritated with his father because he knows that his father plans to go drinking with the soldiers.
It is not explicitly stated that Omovo's father is going out to drink with the soldiers, but it is implied:
- we are told in this excerpt that some soldiers arrived where the narrator lives and proceeded to drink alcohol at the bar: "they ... ordered a calabash of palm wine" and "drank;"
- the narrator's father apparently gets ready to leave home as soon as the soldiers arrive. Indeed, both informations are contained in a single sentence: "Omovo watched them from the window as he waited for his father to go out."
We can guess that Omovo is irritated by this because he and his father are too poor for this kind of expenses. For example, the father doesn't seem to be able to afford a new coat to replace the "shabby" one he's wearing, which is too small for him.