I think that next, Grandpa will
Go to Cyprus - not The Bahamas
Country that he visits
Each of the countries that Grandpa visits - Japan, France, Poland, Germany - has at least one chemical element named after it - Nihonium, Francium (and Gallium), Polonium and Germanium.
Furthermore, Grandpa seems to visit these countries in order of decreasing atomic number.
Nh=113 (which is clued by Nov 3rd), Fr=87, Po=84, Ge=32.
Now, according to this list, there are just two other countries from which the names of chemical elements are derived - Russia (Ruthenium) and Cyprus (Copper) - Americium is debatable for the U.S. but it's generally considered to be named in honour of the Americas.
If Grandpa is to continue the pattern of visiting countries in order of decreasing atomic number, then his next destination must be Cyprus as Ruthenium has atomic number 44 and Copper is 29. I have considered that he may also visit France again, in honour of Gallium, but it makes more sense that he would vary his destination (he's seen France already, okay).
Country that he doesn't visit
As others have noted, the beginning letter of the country that Grandpa doesn't visit is always one before the beginning letter of the country he does visit. So we are looking for a country beginning with B.
Furthermore, the unvisited country is always the first, in alphabetical order, among countries beginning with that same letter (i.e, Fiji in the first country in alphabetical order beginning with 'F'). For the letter 'B' this country is The Bahamas (generally listed as beginning with 'B'. The 'The' is ignored in this alphabetisation).