Well I don't know how old your little guy is... but he looks old enough to start brumating. This is the time of year when many beardies(generally over 1 yr old) start to slow down and get ready for winter. It is very similar to hibernation.
They will start to lay around lethargically, sleep more and not bask as much. Their appetite also tends to dwindle as well.
Here is a great article on the subject... that might help if this is what is going on with your beardie
http://www.beardeddragon.org/articles/brumation/
Does he otherwise seem healthy? How are his poo's?
We could also go over your set up(uvb,temps, etc.) if you want... to rule out any problems with husbandry.