you've been given an excellent advise already, i will summarize:
- temps - you gotta adjust it ( for both frogs)
- cover 3 sided of that small critter keeper, you should do the same for the other one as well
- adjust your dusting schedule, once a week like you do is not enough. dust ca/vit 3 every second feeding, use separate multivitamin powder once a week.

it won't solve your problem but it will sure help and doing so you will avoid problems for the other frog.
now, if it is congenital defect ( very much possible) they there's no much you can do, Ca/vit D3 and multivitamins will help as much as it can be helped. i would like to ask a few questions though
you said you have him for a 3 weeks - how many times he ate, how much per feeding, what size of crickets, poop - how many times, how did it look like? have you noticed anything out of normal during that time other then you mentioned here ( stop eating 4 days ago, small poop a week ago), was he active, hopping no problem after crickets? do you remember how he was kept in a store? does he has any hard lumps on any of the sides?