How to overcome anger with parents who caused financial ruin?

It will be hard to deal with them unless he can forgive them for their past sins against him. They may be quite challenging for him to deal with in any case, if they continue to harangue him over his "lack of success." If they are forgetful, or feeling guilty about his finances, they may always be critical of him. Elderly folks often fixate on certain topics & just talk about them over & over. They often have what seems to be a cut-off point in their memory that doesn't let them remember things that happened after the original event. Good luck to you both.