Can't do anything about the weight gain, they gain as they grow. Just keep feeding him as much as he'll eat, you'll see results.
Cierra ate 4 male roaches (with wings) today. I was taking out roaches to take to my friend Joanne (she's severely allergic, so I feed her dragons with them as a weekly treat), one got loose and Cierra ate it. So I offered her more and she ate 4! I've never seen any young beardie eat males. So, if that's what she wants, that's what she'll get. I put some normal sized into her food dish, see if she eats those, too. She has trouble digesting the small supers and gets runny poops, so I'm glad she's eating roaches! I have 2 colonies, so have lots of those!