If, at any time, there should arise a consensus that he needs to discontinue
warfarin, he will either have to self-inject
heparin or fondaparinux every day for the rest of his life, or he will have to have the mechanical valve replaced with a tissue valve. Neither one of those is a desirable option. But it is probably not the warfarin that is the culprit.
As to what is the problem, you may want to get a second opinion or an outside consultation on your father's overall case.