I'm wondering if anyone will have any reason why Uther, the tiny Beardie, won't grow

So, back in Febuaray, I got my boyfriend a Bearded Dragon (after quite a few months of talking about it), since he's grown a bit, but not much, he was a baby when we got him, still is, and is around 6.5 to 7months old maybe? We didn't get his exact hatch date, but when we got him, he was the largest in his clutch... Now he's about 3 inches with his body size (not indcluding head and tail... so we'll add abt 4inches, since I haven't seen him since May 20th). He had gotten bigger since I babysat back in April for a week, since my bf was leaving town, and I am, and was, only one Uther would let hold, and eat from on a regular basis.

Now, like I said, between the month I babysat him, and the last time I saw him, he has gotten bigger, just... not length wise... We took him to the vet when we first noticed he wasn't growing like we thought he should, and that was begining of April, from the end of Feb. he had gained 1/3 of his body weight in Feb. We're going to get my new Ball Python checked out after he eats and has his first shed, and bringing Uther with us for a follow up weight check (BTW- our Herp vet is AMAZING! Free appt for Uther last time )

He recently got moved to his big terrarium, from a ten gallon, to a 40(?) its 36''x15.5''. Could a small habitat have inhibited his growth? Or could he be a Rankin's Dragon? Or just have dwarfism? (Which the petstore offered to take him back for a replacement Dragon if that was the case, we said no ty, we love him even if he won't grow )

Additonal Info:

-We don't plan on breeding him
-His diet consists of small crickets daily, as many as he'll eat in a 10-15 min period, and collard greens, every other day, the crickets are dusted (well, about 3/4 of them) daily, and once a week, he gets them dusted in vitamin dust, instead of calcium...
-He was impacted the weekend before Memorial Day, cause unknown, so he's getting Reptile Carpet and then we're done with mealworms til he's bigger, if we can, we're switching to silkworms (BTW Impaction was cleared with several warm baths and tummy rubs in two days... he didn't sit still long enough apparently to have a nice long one til his business was done. And has been happy since ^_^
-Temps and humidity kept where they should be, my bf is very... whats the word.. well he always checks so a bit of a very watchful Daddy


I'd post a pic, but none have worked for me yet...Though there is one in my gallery of him.