He may very well be head coaching material but a few years away. This is probably a practice run, so that when he's ready for the job, he's ready for the interview process. Smart move.
My daughter just got an invite from a headhunter to apply to a job that she thinks she's underqualified for. So she hesitated. Advice from her Dad: just do it for the experience! Plus, you might be wrong; let them find out who you are and what you can do, and if they want to take you on and help you develop the skills and knowledge you don't have yet, great! Just be abundantly transparent about all that.