All so-called "weak" nouns in German are masculine, and their "weakness" is their willingness to change their ending by adding "(e)n" whenever they appear in a case other than nominative. This basically means that these nouns all end with "en" when they appear in these other cases, which is important to know because that ending can make these nouns seem plural when they are not (e.g. "der Mensch," "human," "pl. die Menschen," but also sing.akk "den Menschen"). The table below shows several examples, but if you crave further details, you can visit Grimm Grammar.
|(male) customer||der Kunde|