His wife would know best.

He turned away his heavy eyes, saying huskily: "Well, let him come along, then," and relapsed into the clutches of black care, that perhaps prefers to sit behind a horseman, but knows also how to tread close on the heels of people not sufficiently well off to keep horses--like Mr Verloc, for instance.
Winnie, at the shop door, did not see this fatal attendant upon Mr Verloc's walks.

She watched the two figures down the squalid street, one tall and burly, the other slight and short, with a thin neck, and the peaked shoulders raised slightly under the large semi-transparent ears.
The material of their overcoats was the same, their hats were black and round in shape.

Inspired by the similarity of wearing apparel, Mrs Verloc gave rein to her fancy.
"Might be father and son," she said to herself.

She thought also that Mr Verloc was as much of a father as poor Stevie ever had in his life.