he is an older dragon, they do slow down on eating as they get older. usually start slowing down a bit about 1 1/2- 2 year my experience. hit diet should be 90% greens 10% insects at that age. also his basking spot *hot spot* should be 100-105, for younger dragons 110-115. his cool side should be about 75 or so. try raising his basking spot, also give him a warm bath too that may help him. add some electrolytes in the bath. you should be able to get a bottle at your local pet shop. if hes not pooping normally he could be constipated. if so then try some romain lettuce, this should NOT be fed as a normal green but when constipated it helps. make sure hes hydrated, mist him 2 times a day. the bath will rehydrate him also.
good luck
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Beach Beardies
3.3.11 bearded dragons
1.0.0 Turquoise x Sunburst Veiled Chameleon
1.1.0 Sugar Gliders
0.1.0 Miss Queen Athena (Cat)