Answer: Likes comic books
Neil's penchant for incredibly unattractive sweaters is a running joke throughout both films. When Neil says "I am a doctor", Scott says (Not very nicely) "No he's not, he's a psychiatrist". At the end of the movie, Charlie says "That's okay, Neil. You were just denying your inner child."
One of my favourite quotes about Neil:
Laura: All Neil told him was that Santa was more of a feeling. More of a state of mind than an actual person.
Scott: Kind of like Neil.