It means that the dean of the college put him on a special official list recognizing those student's that excelled academically for that year or semester.
From Wikipedia's entry:
A Dean's List is a category of students in a college or university who achieve high grades during their stay in an academic term or academic year. In secondary schools, or high schools, the term Consistent Honor List or Honor Roll is more common, but Dean's List and Consistent Honor List are generally synonymous. It is most often found in North America, but some colleges in Europe use the term, at least at graduation.
Also, at some universities that incorporate multiple colleges, you will be on the college Dean's List. I've heard of at least one school that also has a university Dean's List, so there it would be possible to make both the college and university Dean's Lists.