Is it obligatory to inform one's father that the money invested in stocks and bonds might be haram, and what should be done to purify the money given the uncertainty about the nature of the stocks or whether the bonds were purchased with his own money?

It is an obligation to inform the father of the impermissibility of trading in prohibited stocks. The purification of illicit wealth is achieved by extracting the forbidden portion and disposing of it by spending it on the welfare of Muslims or giving it to the poor and needy.