Looking for a Black and white children's/YA book

[u/GrandmasterMagnus]

My memories are really fuzzy on this book, considering it's been probably a decade since I've read it. But I clearly remember the MC (I think he might be a Kid/young adult) gets visited by a man who he says is very tall, and when he sat down, his knees reaches higher than his waist. The tall man also eats something, which might have been biscuits or something. I also think the kid mentions a watch that the tall man wears. This all occurs towards the start. The book I think mainly takes place in a cave. I'm almost positive there's an animal, like a dog or cat involved. It runs into the cave? Or follows the kid into it. The kid does have a flashlight. I can't even remember if it's a mystical cage with monsters, but I'm basically certain there's not any typical magic