hi
i'm italian and like to travel too , i think some of the best places in europe are SPAIN ( because is a sunny place ,i love sunny too)

, i agree with amsterdam ,never been there ,but many of myfriends visit and they say is very nice and clean !
anyway .. was a generic answers ,becouse you need to see what are the interest of your son and friend! maybe they don't like history ,and just want go around and have fun ?
if so ..they could go in SARDEGNA island, very good place for the summer,
ciao