I agree with MISHKE. But serious cases of MBD should be dealt with by the vet.
If it doesnt get much better or continues even after MISHKE's recommendations it may be a good idea to see a herp vet.

Also, to try and get him back up to healthy weight, you should try feeding him meal worms, wax worms, butter worms etc. These should be fed on top of the normal cricket diet he is getting. You could be adding calcium powder to the crickets and the worms as well using a sort of "shake and bake" method to coat them in the powder.

I hope all goes well and good luck