You ask some very good questions about your adventurous grandfather: Why was he an adventurer? What did he learn about life being an adventurer? Then you say, "I don't know the answer to those questions."
That is the crux of the matter, I think. You don't "know" (no one does, or ever will, perhaps not even your grandfather), but you can imagine. You CAN allow yourself the freedom to speculate, assume, and eventually create your own impression of the character your grandfather was, along with what his thoughts and feelings might have been.
I can hear that you're on the verge of imagining what your life would have been like if you were in your grandfather's shoes, living out of a suitcase and visiting the far corners of the world -- so go ahead and cross that verge, and imagine yourself as your grandfather. You know his "outsides," his facts, and from them you can create your best guess as to his insides. It might help to find out a few things about the countries he saw, but not overly, because you're creating a character rather than writing a travelogue.
A theme doesn't have to be confined to a single sentence--that's just a technique for focus. I'd suggest free-writing a paragraph about what it would feel like for a guy to live life out of a suitcase for 50 years--ups and downs, motivations, promises, regrets, amours, anything you might think of. Your theme will be the parts of the paragraph you keep keep returning to as you write more about his life. In other words, your theme will be his unique combination of traits, as informed by your research and filtered through your speculations and creativity.
Your book sounds like a lot of fun to write! Enjoy!