Hi, mealworms are not all that nutritious by themselves your right. I don't like mealworms much, they are very high in chiten, higher than supers, and harder for them to digest. I would use supers they usually love them, I have some that have lived off them for years that do not touch crickets. But most do like the crickets better. "There are some exceptions", so make them healthy by feeding your feeder bugs well. So they are nice and nutrious for your lizards. What boost said is good to , with a picky eater try every other day for awhile , make him get a bit hungry :0)
As far as him being SKINNY and not gaining any weight...
First let me ask are your temps up there at atleast 105 degrees at the basking site, and high 80's to 90 in the rest of tank?
**** PARASITES
Have you had a fecal done on the male to check for worms/ parasites?
Have you turned him over tummy side towards you in palm and pour some mineral oil or olive oil on his tummy and smeared it around and watched with a magnifying glass to see if tiny black/ tan or red spots begin to appear?
These are just a couple things to try. Hope all goes well and you can BEEF that boy up. 
Suncharmers kids
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PHEve / Eve