Temperature Control/Thermostat Advice?

I currently have a very stunted growth wise 6-month-old beardie I rescued last week. He's my first beaded dragon, so I'm new to a lot of stuff. Mushu (the beardie) is only about 9 inches long including his tail, his body is less than five inches. Right now I have him in a 5.5 gallon tank, with UVB and heat lamps, with a basking log in the form of a twisty piece of desert wood, in a spot that gets direct sunlight for a few hours in the morning, and bright light throughout the day.

My main concern is regulating the temperature. The thermometer and humidity gauge are in the middle of the back glass between the two regions of the tank, and read between 78-85 degrees for most of the day. However, with summer, the ac will be on, and I'm worried it will be difficult to keep the tank warm enough for him. I'm looking for advice on some kind of thermostat for the tank, that will keep the lights on, and adjust the temperature in the tank by lowering or raising the power of the lights when I'm out during the day. I also do not trust the thermometer in the tank because it's the crappy kind from the pet shop that don't work if they get wet, but they were in his tank when I got him, and I was hoping to have a better solution for monitoring temperature by now.

It would also be nice to have something that I can use as he grows, and can move from tank to tank for at least the next year depending on the price.

Any advice would help, thanks so much!

Hi. You can use a temp gun or a digital with a wire probe. You definitely need to get him a bigger tank ASAP. Its impossible to get the correct temps in a tank so small. I bought all 3 breeder 40 gallons for 50.00 or less of Craigslist. Also I know that petco has sales on their tanks 1$ per gallon. So just keep looking you'll find one.

Ive heard many good things about herpstat
Your want a dimming thermostat AND a good thermometer (it's not always a good idea to rely on the stat for measuring).

A bigger enclosure is absolutly a must.