I am wondering if it is a caching issue. The student may be getting a copy of the page from before he did all the problems. Check if YOU see the grades for those problems, and ask him to reload the page if you do.
Another possibility is that the student did some of the problems, then logged out (or timed out) and then logged back in as a guest. You might see of any of the guest accounts have answers recorded for those problems (and see if the login times correspond to when he claimed he was working). You can find the login.log file in the course's "logs" directory using the File Manager (go up a level and then down into logs).
Otherwise, check the past answers, as Brian suggests.