If one of the elements of his work involves forming interest-based contractual agreements (e.g. drafting them) or ask the customer to pay the interest, then his earnings pertaining to that part of his work is unlawful.
There is no verse in the Holy Quran that appears to have a cure for this habit. However, getting married and fulfilling your desires in the permissible way is one of the best ways to stop committing this prohibited act. It was reported that whoever gets married, two thirds of his religion is guarded.