Thank you all so much for your answers! I am a bit concerned about the amount of work that this kind of hobby takes, but I do not want to ask him too much about it since I don't want to ruin the surprise. My grandfather used to build planes and loved it, so I am hoping that he will enjoy it just as much.

I was thinking of getting him the engine, a radio controller, and a basic tool set and some modeling knives (and also include a builder's mat?)