A 20 gallon is perfectly fine for a 10" beardie but not for long. It sounds like he's growing very well in your care but will need an upgrade soon. Keep an eye on Craig's list for a large tank , you might find a good deal. Or see if your hubby might be motivated for a fun project to build a big enclosure.
Karrie is right about the lights, be sure to get a good
uvb bulb. Some of the coil/compacts can actually be used in a smaller tank with smaller dragons but again, you need to upgrade very soon. If it's warm out whee you live you can also build or buy an outdoor cage so he can get real sunlight.