Apple automatically selecting wi-fi connection

hi - my dad has a problem with his mac, i think its osx.

for a while the only available wireless network was his own belkin connection, and that was auto-selected when he launched his browser.
now there is a choice of wifi connections and he has to manually select his own from the pull down menu

is there a way of automatically selecting his own wireless connection?

thanks.

 

network preferences

airport

join preferred networks first

have his network as the only preferred network

Airport likes to join the network with the greatest signal first normally