Broken record here!!! Call your vet - or better yet, take the dog in. I know I always preach the same sermon but you'll know the answer then and have something to take care of the problem.
Cavaliers have such heavy ears that any moisture that gets into the ears is held in. (They aren't the only breed that does this) So ear infections aren't foreign for them. And if it has any kind of "off" smell it's even more important to get him seen!
RIP Wallis, John Robert, Tibby & Pip
Mom to JoJo, Roxie & Linguini (Cavaliers)
and my birds Moji (Senegal) and Walter (Blue Front Amazon)