What sort of lights have you got for him. Without a proper uv light, bearded dragons can/ and most likely will develop problems such as MBD(metabolic bone disorder) which can produce symptoms of lethargy and what not. After time uv lights will reduce the amount of uv they produce, I think it is best to replace the uv about once every year. what brand uv bulb are you using? the spiral ones have had some bad rep in the past, I stick with a UVB tube and its important to note that the percentage written on the box should say UVB 10% as 5% isn't enough. Lethargy could also indicate a number of other problems, is your bearded eating every day, and drinking.
To be honest, I have never seen my beardeds drink from there water bowl, I don't know why but I spray water on the veggies just to keep their fluids up.