The Song dynasty book (960–1279) "Hundred Family Surnames" lists the most common Chinese surnames. I would pick from one of the top ten most common, but I would not use the most common because that would be like using
John Smith as a pseudonym.
https://en.wikipedia.org/wiki/Hundred_Family_Surnames
For the first name, he might choose a name that doesn't exactly reflect who he is but is close enough to not seem unlikely. He wouldn't want a name that seems royal, but, if anything in his manners or appearance suggests that he is from a slightly higher station, then he might want a name that makes sense for his assumed station. I'm not familiar with the naming conventions during that era, but maybe something like
Qing,
Shui, or
Yun would be okay?