Your dragon is under stress because of the new home. Hence the darker color and displaying. Checking the temps is a good ideal as you will need stronger lights to maintain normal basking temps etc. He will also need a week or two to grow accustomed to his new home. If he starts to lose weight, it may be better to return him to his old cage and give him a few more months ot grow before moving him back into his new adult home.
To help reduce the stress of the new home, block all sides ex cept the front with black paper and leave him alone. Treat him as though you just bought him and let him get u sed to his new home. Once he is back to eating regularly, you can slowly uncover sections of his cage and start handling him for brief periods of times. If he starts to show signs of stress, stop the handling and give him a few days. REcover the last uncovered part of the cage if he doesn't resume eating after a few days.
ALso, watch for signs of parasites (watery stools, really really smelly stools, traces of blood in stools etc.) If any are seen, or if there is any concern, take a fresh stool sample (refrigerate if unable to take it to the vet the day it's dropped) to a vet for a fecal.
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PHLdyPayne