Engine won't stay running when trying to start

This is a diesel truck with a Bosch fuel system. When turning key on, the engine will not stay running. You have to hold key in start position for approx. 30 seconds-several times. (Just started doing this) A couplr of opinions have said fuel sensor, manifold gasket, etc. I would really appreciate a professional opinion/. And I am one of the ones that cannot afford professional advice. I am asking for my elderly father in law but I think if you could help him he might show his gratitude with a small donation (on fixed income) Thank you so much for your time and help re this matter.