How old is he? How long ago did his grandfather die? Sounds as though he is worrying and needs some comfort at night, not ideal for you though. If talking about what's on his mind doesn't work, you could try role playing. So you play along with his pretend games and see what happens... or play with teddies/figures - as long as he is leading the play, something will probably come out (if you have a hunch about what it is, you could say, so this teddy is daddy, this one is you... then let him lead). Children often process anxiety through play.

How long has it been going on? Could you see it as a tricky phase and let it happen until he's settled? I hear you though, it's hard enough at the best of times to get any adult time... or decent sleep.