Is it ethical to offer a student a job during the final weeks of his thesis if you are partly grading it?
It is perfectly ethical to inform a student that you are currently supervising about open positions, especially if the student is about to complete the thesis and is a good fit to the position.

It is of course not ethical to let the student's action upon this information affect the grading of the thesis. That's for you to take care of, and I guess you were already aware of that.

If you have any concerns about putting pressure onto the student, an alternative way would be to have someone else inform the student about the open position. You write that you supervise the student's thesis, but your institute has the open position. 

If that means that someone other than you is directly responsible for the open position, than just ask that person to contact the student about the position. That will probably make the student feel less guilty with respect to you if he or she doesn't apply for the position.