Did his previous owner get him as a baby or has he had many owners in the past? What did his previous owner feed him?
What size tank and what kind of top do you have?
What do you use for substrate?
What kind of UVB lighting do you use, coil or tube?
Is it mounted inside the tank or on top of a mesh screen?
If mounted inside do you have a reflector and how long?
What kind of basking light do you use?
How close can he get to his lights (4", 6", 8", etc.)?
Temperature of basking spot, cool side and ambient warm side?
What do you use to measure the temperature?
Do you have a Hydro meter to check his humidity, if so what is the humidity range?
Does he have a spot in his tank to hide and a way to climb?
What size are the crickets?
What kind of vegetables are you offering?
Are you using any vitamins, if so which ones and include the brand?
Do you know how much he weighs, if not you can get a cheap kitchen scale for most stores such as Walmart. It is best to weigh in grams, grams is more accurate and a find compare to ounces and pounds.
Can you get in contact with the previous owner?
I am far from being an expert, but I saw your post and thought I would ask questions more experience people may want to know from the beginning. Please take pictures of his enclosure and of him, you can post using this link:
https://www.beardeddragon.org/useruploads/
From the little bit I do know, it may be the change in environment, 2 to 3 years with one owner is a while and being switched could be very stressful. If he is stressed it may be best to not handle him for a few days, only handle him when necessary. I have some links in some of my previous posts that I found to be very useful, I will find them and post them for you to refer to.