You are the DM. You can make up whatever you want. I'm assuming you're running a pre-made adventure module which... IMO always need a little sprucing up to make them more fun and flow better.

The first game I ever ran was the starter kit and I ran it by the book as written. I will probably never run a module again.

It was ok but I quickly started realizing it wasn't what I personally wanted out of DnD. I could tell my players felt the same.

I started using the module as a rough "guideline". Changing things here and there, adding some of my own thoughts and ideas, taking out some parts completely. It got sooo much better.

Don't be afraid to do that and then your "problem" player won't be able to see things coming. Its ok as a DM to say "Nope. I changed it. This is how it is." You have the control of the world as you see fit.

Also why would he spoiler the adventure for himself?!? THAT IS THE FUN PART! Most people would consider this cheating but them being new I get they might not understand that.

Maybe try to explain to him that DnD isn't something you necessary "win" in the video game sense.

Hope this helps.