My best friend's disturbing diet?

Since we were really little a good friend of mine has always eaten these things only: peanut er crackers, trix rabbit yogurt, pretzels, garlic bread, and occasionally a peanut er sandwich. Were halfway through high school and I have yet to see him eat anything else ever. I've come to accept it because he's my best friend like I said, but I will always wonder if this will lead to any serious health problems? He isn't obese at all, in fact he's almost too skinny and cannot physically exert himself for too long without becoming winded. Any thoughts?