The Annoying Grandson Problem

I've been babysitting my grandson every weekend for a few months now. He's six and absolutely wild. His parents work long hours, so it was agreed that I'd help out until they can sort out childcare. He loves coloring books, which is great, but he also has a habit of tearing the pages out. When this happens, he'll hide them or try to sneak them back home in his backpack, thinking no one will notice. This happened again last week when I took him to the park. He tore out the page with the picture of the cat and tried to stuff it into a flower pot. I asked him why he does this and he shrugged, saying he likes the drawings but doesn't want to ruin the book by coloring inside the lines or staying neat. We've gone over rules about respecting books and keeping them intact, but nothing seems to stick with him. Then there's his obsession with playing pretend that everything is a toy car. This morning, he even tried to put my phone in his toy car and roll it around on the floor like it was actually moving. It was amusing at first, but now I'm getting frustrated because I need to keep an eye on him constantly. At times, I feel like I am not cut out for this role of a babysitter. Am I being too strict? Should I just let him be a kid and let the books get ruined, or should I try another approach to teaching him responsibility? What would you do in my shoes?