Hi, I'd advise keeping an open mind about the way names appear in old handwritten records and the transcriptions of them (the typed version). The candidate I posted was in the Danish census 1860, which is a strike against him, for sure, IF your John was in California in 1855, which is possible but not established solidly. Let us know the first solid identification (earliest year) you've found.
On the other hand, the distinction of sen/son is not impressive evidence against the candidate. If John was indeed Danish, his name will likely appear sen, regardless of the spelling he used once he had immigrated to USA. Even if John used son in Denmark, which is dubious, the Danish census taker might not ask John's spelling preference! Sen is "the Danish way." If you shut down further search due to sen/son difference, you may be shooting yourself in the foot.
His name could also appear as Johan, Johannes, Jens, or Hans. William could be Wilhelm or Vilhelm or other. His last name could appear in a given recorrd as a place name (or other, e.g., an occupation name). His patronymic name of Hanson could be based on a father's first name of Hans (Hans' son) or he might be shown with his father's patronymic; various families dropped patronymic naming at different times through the 1800s. And immigration was a catalyst for change in itself. An immigrant might adopt one of those variations different from what was used in Danish records. The same person could appear in records with any or all the variations. So, the sen/son variation is just the tip of the iceberg of what you must be aware of as a possiblity.
Please post the records you have found to date. Instead of talking about a record, please show it (type up the detailed content and specify the source). Explain why you think he is Danish if you have reasons above or beyond public records.
Thanks!