I think that in a case like this, I would have first an honest conversation with myself, is the complaint a possibility in that you have a firm mattress? Old mattress complaints are not usually a hard mattress. Do you think that there is a different reason for canceling and he is just making up a reason that won't be too much of a negative for you? Or has the guest been a problem for 3 days and you are grateful he is leaving early?
After I figured out what my thoughts were then I would make a determination based on that decision. A problem guest or a considerate guest that who tried but after 3 nights is not sleeping well.