Im my opinion... I would not put that heater in there. In such small spaces, even a slight change in temp could make the water heat up too fast... therefore cooking your little guy! I would continue using the heating pad until you save up for your 10 gallon.
( Besides, the system your using now is working... So why take the risk!) But I am not an expert... So see what others say :)

OB is right, a 7.5 watt heater is too big for a 1 gal aquarium, OB is also right, wait until you upgrade him to that 10 gal tank.

Save the money you would have spent on a heater and get that 10 gallon. The heat could rise way too fast like the others have said, I wouldnt chance it.

Thanks, everyone. This just gives me more incentive to get that 10 gallon. I'm so concerned the heating pad will be a fire hazard.

Where do you live Pureza? I have a 2.5 gallon with no heater.. and these conditions are fine for my betta.. hes over a year old now. When I change the water... i feel the temp of the water and make sure its a little above room temp. Then I fill the tank. I live in CA so my house is always kinda warm... and if its not I have the heater on! ;) Anyways, the tank has a light built into the top that is the size of a night-light bulb. I think that also keeps the water warm too. Depending on your situation.. you might not even have to use that heating pad.

I live in Mississippi. We have a very cold house, central air is usually set to 72 degrees. Before I put the pad under him, he wouldn't do much. Now he's swimming around and seems to be happier.

I have 2 15w lights on my 10g tank. They seem to heat the water up about 4 degrees (Farenheit) during the day, but it takes about 8 hours to do that. Perhaps all you need is an incandescent light shining down on the water. Or into the side of the tank.

Recall the incandenscent (screw in) lights give off much more heat than flourescent ones do.