I'd go with diamc on that. I know Odahviing has seriously slowed down her ALWAYS SHEDDING OH MAI GOSH EAT ALL THE THINGS growth rate. She's just turned seven months and started to slow down around six. Now I have to ease her back because I'm still feeding her like she's growing that way and she's getting, ahem, a little fat. I know the actual size of the beardie can vary a lot, but Odahviing is about 16 inches right now.
When you turn him to face away from you and bring his tail toward his head, holding toward the base, do you see a pair of bulges running up parallel to the midsection at the base? They'd be just little bumps almost like tiny straight growths under his scales, basically because that's exactly what they are.
You can just put him belly down on the palm of your non dominant hand, his head on your fingers, so your thumb and pink gently can keep him in place if he gets wiggly, and just take your other hand and place your thumb on the under side of his tail, about a little less then a quarter of the way from his vent, and gently bring it back toward his back until the tail is at nearly a 90 degree angle up from his body. Then let the tail curl back naturally a little more toward his back, don't worry, it doesn't hurt! There at the base above his vent and on his tail is where you'll see the pair of bumps if he's a boy!