poor Louie, sounds like he has an ear infection. There are so many causes it's hard to say how he got it. What you're probably seeing is excess wax that builds up from infection.
An evening vet appointment will be fine. Even though it's bothering him, he'll be ok til tomorrow. The vet wll also be able to look in there and see if there is anything in there, like a small seed or something.
Since you're already going to the vet, I'd leave it alone till the vet sees it so he/she can get a good look in the ear.
Let us know what you find out!