Didn't read this below before my post above.
I know it hard but try not to compare your boys. The older is possibly having a hard time trying to keep up with his brother. Still hard to tell if he does or doesn't have dyslexia without an evaluation from a specialist because there is a HUGE variation on when a kid is developmentally ready to read. And it sounds like he is still ahead of the curve.
That said, if dyslexia is the problem early intervention is key. Try finding someone specifically used to working with dyslexic kids. The local private school for dyslexic kids in my area takes kids as early as pre-K. If private schools will take kids that young, it must be possible to find someone to test him.
