My niece was 8 when her world came crashing down. First her sister told her, but my 8 year old niece was not convinced and cited photos of Santa as evidence that he existed. Eventually her friends told her and she went to her mother and of course my sister had to tell her the truth. She was actually pretty upset that they kept the gig going for that long.
I would say just tell him the truth after this Christmas.
Tell him that
Krampus is definitely real though!