The simple way is for the two of you to come up with one of the future topics that will occur in due course. Work with him to develop a lecture, just as you would work alone to develop it normally, but with both of you present. He then presents the lecture himself. Be sure to anticipate questions that other students will ask so that he is prepared for that and can be flexible. Preparing notes for the talk using a Hipster PDA might be especially useful and would in addition demonstrate that technique. At one time college debate teams used this concept to prepare.
The somewhat more complex way, but which would be easier on some students, is to prepare just as suggested above, but then team-teach that lecture, with each of you responsible for some part of the process.
For me, the best way is to pair-teach the lecture. This is a concept that a colleague and I developed. One of you (likely the student) starts the lecture. The current person is the driver and is in control. The second person is the navigator and is responsible for watching over the process in an aware way. If there is any stall in the action, perhaps a student question, the roles switch, with the navigator taking the lead and the former driver watching over. One of the tasks of the navigator at any moment is to be aware of the class response to the driver's words/actions and can step in whenever there seems to be puzzlement.
Either of the second two methods makes it easier for first time lecturers, since they know you "have their back" if things get tight.
I've experienced the first of these as an undergraduate and used the two others in teaching, the last one extensively.
A somewhat lesser idea is to prepare for a topic not normally in the course and prepare for it. A good time for the presentation may be on the last day of the course, especially when the final exam has already happened. Tension may be less and you often need something go pique the interest of the class on the last day anyway. Any of the three methods suggested above could be used, of course.
I would personally choose the method based on the personality of the person and his relation to his fellow students. If it is especially open and friendly, the first might be less fraught/scary than otherwise.
You can use face to face contact (office hours) or email, etc., to develop the lecture. You can give the student an outline, or ask for one instead. Lots of variation is possible, but you want some sense that the student is well prepared both technically and personally.