A leader is not the one who sells the most on your team. It is the one who helps others get results even when you are not there.
Define what leading means in your organization
If you have not written it down, each person interprets it their own way. Write three concrete behaviors you expect from a leader: for example, training their team every week, following up on their goals and sharing what they learn.
Give real responsibility
Nobody learns to lead just by listening. Give the person a small group, a concrete goal and the freedom to decide within clear limits.
Accompany, do not control
Ask before correcting. When people find the answer themselves, next time they no longer need to call you.