I would encourage you to conduct a counseling if only because AR 600-8-19 specifically states a Soldier who is fully eligible will be counseled. Simply tell the Soldier he is doing a great job and, if his performance is still to standard when he resolves the weight issue, you will consider recommending him for promotion at that time. If nothing else use it as a motivator. You may also discover things you did not realize by conducting this counseling. For example, the Soldier may not be interested in being promoted.
Read The Mentor: Everything you need to know about leadership and counseling for more information about Counseling, Leadership, Corrective Training, and Separations in the Army.