If he really only weighs 83 gr. he couldn't have weighted that much more when you got him.
That means that at 6 1/2 month old he must have weighed 90 gr. or so. I bet that he was a hatchling that just wouldn't eat for the breeder and was eventually assist or force fed a couple of meals. Most likely "pinkies", since people will assist/force smaller meals.
There just isn't another good explanation for SUCH a low weight for the age.
I "assume" the breeder assist fed a few small meals and then decided to un-load that animal. It should have NEVER been sold. Esp. to someone with little experience. The added stress of being carted around to a show, moved to a new home etc, didn't help.
No wonder this animal is lethargic. Having had trouble swallowing a hopper and also not putting up much of a fight during assist feed also shows that this is a weak animal. Most likely due to the lack of nutrition during its life so far.
Just like everyone else said, make sure you have husbandry SPOT ON. That includes lots of privacy, blacking out sides and back of enclosure, hides, etc.
It is good that he was at least willing to take a meal (even if he didn't get it down yet). If the assist feeding may have given him a boost, he may be willing to take another hopper and get it down next time. Give a break with NO STRESS (no soaking, moving, handling) and then try the live hopper again and cross your fingers.
I think there is hope. And you are doing great trying to help this little one and seeking the right answers. So kudos to you.
As for the breeder selling such an animal ? Shame on him !!