For him, it means that there are some unanswered questions...something that his grandmother could have answered for him.
If I was him, I would use the dream to seek out the answer...it may come from personal belongings she left behind or through other conversations she had with loved ones.
Is he seeking the answer to some big life question? It could be that a memory of her or a conversation they shared is telling him which way to go or what to do.
I never question dreams. I always seek out the answer to them. Especially when it comes down to people who I have a special connection to. My grandmother, who died several years ago, appears to me saying "first you make a roux" everytime I start to cook...