Hi there
I'm new to these forums but not to reptiles.
Reptiles are known to lower their metabolism when their habitat is hostile. They can slow or even stop growth. This shouldn't be the case since you gave him food since you have him, but his genetics could be playing a small (or big) role, slowing his growth because the type of food wasn't changing (size of prey). If it is indeed the case, then once you start giving him the proper care his metabolism should resume and start growing again. Of course, failing to adquire proper nutrients in early stages of life could lead to permanent changes in the animal, like a small size (although not so small), mental disorders or behavior anormalities.
But, genetics could also be the ONLY problem. Intense interbreeding can cause (and will cause for certain in the future) genetic issues. Lethal homozygosity and other body malformities are easily spotted by breeders but other genetic disorders (and a great part of them) are hard to aknowledge and probably will go unnoticed until adulthood.
Now, I'm not saying this is the case and i'm not accussing the person or shop who sold you that animal of anything because if your ball is indeed genetically malformed, there's no way anyone could've know.
You can only get him the proper habitat requirements, give him bigger prey and hope for the best.
If he's not ill but doesn't grow, well, you'll have a unique dwarf ball python and you should be happy because he survived.


Regarding the humidity, if the moss dries, then buy a small tupper with a lid, make a hole with a knife or a tin-lead welder and place wet (but not too wet) sphagnum moss inside. Your guy should hide in it and that way you can give him different gradients of humidity in his cage. But remember to change the moss every week because if you don't it will start to rot.