Can you put his tank in a warm room? Like, my Bowser is in a 2.5g on my desk with the desklamp and the apartment's heater is under the desk so the two keep his tank pretty warm. Somwtimes with his heater the temp reach 85F (which is the extent of the heat he gets) You don't want to stress him out with temp fluctuations but if you can get him in a warm room or place heat packs around his tank. . That could help.
WalMart sells small internal heaters for as low as 11.98. I know not everybody has this money- I'm in a similar situation myself but if you could sell a couple dvd's or cd's or hold a lemonade/bakesale stand (have the younger kids do it) to raise a couple dollars.
**Edit-- I'd return the heater then. A heater should have the tank warmed after at least 2 hourrs. What's the wattage of the heater? And what size tank do you have?