Translation:His parents do not have a swimming pool.
'Basen' is masculine.
The rules for choosing the ending in genitive singular are (taken from Polish wikipedia page https://pl.wikipedia.org/wiki/Deklinacja_(językoznawstwo) ):
Masculine animate and neutral nouns: -a (e.g. 'kot'->'kota' (cat, masculine animate) or 'drzewo'->'drzewa' (tree, neutral))
Masculine inanimate nouns: -a OR -u (e.g. 'bat'-> 'bata' (whip), 'basen' -> 'basenu')
Feminine nouns: -y OR -i (e.g. 'mysz'->'myszy' (mouse), 'ziemia'->'ziemi' (earth) )
As always, there are exceptions. In particular masculine nouns ending in -a in nominative case get -y/-i in genitive (e.g. 'poeta'->'poety' (poet)).