In a small town, there are certain people who earn their nickname like "Grandpa", "Nanny", or "Pops". People find it an endearing terminology and everyone just goes along with it. I don't think it is strange at all for the friend, the Sheriff, or anyone else to call the fella by his "name". It is how they know him and know of him.
There is a lot of speculation on the relationship between the GGF and friend. Another small town thing, imo. It was nothing odd to see fishing buddies who were years apart because they held the same interest and one was just handy to do the digging for worms. And maybe...just maybe...IR doesn't have a grandpa and this is a surrogate of sorts for him. That happens, too.
But doesn't ggp live 2 1/2 hours away from the Sheriff's territory? I feel like he calls him "the grandfather" instead of releasing his name. Is it possible the sheriff is saying the three are POIs because they were there, but not suspects at this time because he's hoping a member of the public will come forward with incriminating information? If someone has been deemed "solid" by the sheriff, someone might be more likely to discount some little anecdote or detail as unimportant. If they're POIs, anyone whose heard one of them yell at their kid on the playground might come forward. Just trying to understand the reason for the gradual change in description over the last few weeks.