Well you need to ask your boyfriend what kind of fish he likes to look at. Tropical fish need to be in waters ranging from 72-78 degrees. So they will need a heater to keep the temp constant. Anything else (Goldfish and Koi) like to be in temperatures from 63-72 degrees, so they don't need a heater. With Tropical fish you need to pay attention to the temperament, Community, Semi Aggressive, and Aggressive. I find that a lot of people get tired of the aggressive fish, after a while their fish keep killing new additions to the tank. So their stuck with two 3" fish in a 55gallon tank. Not too interesting. I have a planted community Rainbow tank with some community catfish and freshwater shrimp. They're beautiful and have lots of movement in the tank, so there is always something to watch! Oh yeah, live plants need special care as well.
Is he interested in saltwater? If so I would HIGHLY recommend that he start with a freshwater tank until he gets used to the regular maintenance required to upkeep a tank, then move on to a SW tank when he's ready. REMEMBER: SW tanks should be over 29 gallons unless you really know what your doing. A 10 gallon tank is not the right size for a SW beginner. With both tanks, you should be doing a 20% water change, via gravel vac., in the tank every two weeks. Water quality constantly deteriorates, so the only way to help the fish is to remove some old and replace with new water. (Helps reduce Nitrates)
You can find Tank kits at local pet stores that have everything you need to start around $60 (10gal) all you need to do is pick the decorations and gravel color. Have fun!
I don't automatically recommend starting with coldwater fish because once your tank cycles, and you want something else, what are you going to do with your "starter fish"? Flush them? NO way! Just read up on "New Tank Syndrome" when you set up your tank. Do not overstock the tank too soon.
I work at a pet store.