One and a half year old snake, 50 grams?

[Eric Alan]

Obviously there's a slight deficiency in his conditions but do you think this could be the cause for no weight gain whatsoever since birth? Or is it indeed underfeeding

Slight deficiency? You have a 55 gram 1+ year old ball python that's never had a good shed and has had prolapsed hemipenes for 2 months. That in itself screams that he needs to be cared for differently.

Regardless of the food issue, getting control of your husbandry is the low hanging fruit that you can (and should) correct immediately. There could be other issues, but right now you can only control what is in your control. Then you can move on to other things.