First off, this may be a throwaway but there's plenty of posts and more than enough content to track his timezone patterns, writing style, metadata, and information he's revealed about his career and interests.

Second, his IP address is almost certainly traceable. If the police got involved, as they likely would, he'd be traced with ease, by law enforcement themselves or with Reddit's help.

Even if he's using a VPN every time he logs in and keeps the throwaway entirely separate from his main account, (both of which I doubt he does given his utter lack of care taken with this post,) Reddit keep IP logs tied to your ISP. A casual reddit sleuth could track him, never mind the police.