No, you can't assume the match is on the paternal line.
As a female, you have 2 Xs – one from each parent.
As a male, your match has only one X – from his mother.
X-chromosome inheritance is not linear. If you are related to his maternal great grandfather, for example, you will also not share any X-DNA.
The following is a nice chart which highlights the ancestors from which a man inherits X-DNA:
Source: The Genetic Genealogist
You could easily be related via ancestors in any of the white areas.
Also, for a distant match like this, even if you are related via an ancestor with whom you might expect to share X-DNA, you might not share any X-DNA purely by chance. DNA inheritance is random, with genetic recombination occuring in each generation. After several generations, the 'common X-DNA' might have been diluted such that matching algorithms cannot identify it.
In summary, a negative X-DNA match does not rule out relationship to any of his maternal ancestors. However, a positive X-DNA match would be suggestive that the match was on his maternal side.