Nix....fish are fine being left a few days each week without food and will probably benifit from it. Just don't add more on sunday to compensate for the days you've missed.
Lore.... tropical fish are actually easier to maintain than goldfish and, apart from a heater, the equipment is the same.
Goldfish are messy fish which require large filters and good water changes if you are to keep them properly.
Why not try the local papers....second hand aquariums are always for sale and a 24" x 12" x 15" tank is an ideal starting point.
Get your son to have a read up on how the filter works and why it's needed and get him to talk to a reputable aquarium shop in your area.
A good forum to join is
http://www.theaquaticroom.com/forum/index.php where you can ask questions and you will get decent answers.
Oh....and with regards to "snags" comments on the guppies above, they are not hardy, easy maintenance fish any more due to years of inbreeding !!!!!!!!!!!!!