It is grammatically correct, but is it what you mean? You use 'student' and 'thesis' in the singular, so you are referring to one student. For a professor to have only one previous graduate student would be unusual. Not impossible, but unusual. You also use 'copies' in the plural, so you are saying that he has multiple copies of the same thesis. Again, unusual, not impossible, but unusual.

Grammatically : yes. Logically: no I agree with Gary ( in fact, I find that I always agree with what Gary says...). The grammar - in itself - is fine, but it's hard to imagine a situation in which you would say this. The present simple 'keeps' suggests that this is the professor's regular habit. But I have trouble seeing how a professor can make a regular habit of keeping multiple copies of a single thesis written by one individual student. This doesn't make a lot of sense. An equally correct but far more likely version of this sentence would be this: "The professor keeps copies of his previous students' theses." This means that the professor always keeps copies of the theses (plural) written by his students (plural). As something which the professor does on a regular basis, this makes far more sense.