causes - just about anything, whatever is consumed and not digested can be a cause of impaction, the less moisture content having foods have the more chances. and all non food items with moss being nr1. even if a frog been fed outside of the enclosure moss can get ingested when a frog is shedding.
treatment - home remedies, such as warm baths with honey or electrolytes, but mostly warm water that do a trick. there're specific vet treatment that you can go for if baths are not helping, and surgery as a past resort in worst cases.

helpful advise - get all the patience you can get so just do baths and very gently(!) massage his sides and please keep us updated.