My grandson is 17 yrs old and will not eat anything but popcorn and peanut butter. What will happen to him health wise. Thank you

Mineral and nutrient deficiency depending on what he misses out (sorry I'm no food expert) when you say anything apart form peanut butter does this mean he won't eat things like peanut butter sandwhiches, his food dependency will be a psycological thing so take him to somewhere that specialises in that area