That helps. One big reason that your dragon may be lethargic is that he is cold. The warmest part of the cage should be kept at 95-100* F, with the coolest part at 75* to 82*F. Without these temperatures, a frilled dragon is going to be slow, and will eventually get sick. You're right that the species is arboreal. They should not be kept with bearded dragons. The two species have different requirements for temperatures, humidity, types of cage, and diet. Putting two different species in a cage together can also lead to greater chances of disease.
Your UVB light is not powerful enough. With a 5.0, it is likely that Metabolic bone Disease will develop. I recommend a Reptisun 10.0 with a straight tube bulb. The bulbs have to be changed every 6 months, even though they still look fine. They don't put out enough UVB rays after that to be effective.
The terrarium you have now is fine for awhile. At adulthood, he'll need a much bigger cage. I recommend this site for information on the species in the wild and for captive care:Frilled dragon
It would also be a good idea to schedule an appointment with a reptile vet. I recommend that all new reptiles be examined by a vet shortly after purchase. While it's difficult to tell just from a photo, yours looks like he is too thin. He may have parasites or another condition which needs to be treated. This link will take you to a directory of reptile vets:http://www.anapsid.org/vets/index.html#vetlist
Try the Pedialyte soaks and warmer temperatures this weekend. If your dragon doesn't show improved activity level and appetite by Monday, you'll need to see a vet. If you have more questions, just let me know by clicking on REPLY.