Amy,
I can see that you are concerned that your Dad's buying is masking other more serious issues and that if it continues he may end up in financial trouble.This may or may not be the case but with everything else I understand your concern.It's easy to see that you care.
I think that if it were me I'd casually mention the five year plan,maybe ask him what he plans now and tell him that you love him and that you are there for him.