About the only way you can get a valid diagnosis is through testing. Our son's was diagnosed when he was six by a neuropsychologist. The speech therapist began using the Wilson Reading Program, and his reading and decoding skills exploded. There are several programs like Wilson that are a better fit for readers with dyslexia, so that might be an option for actually teaching the skills - although your older son may feel it is a backwards move and resist.

~Lisa
http://www.lisaabeyta.wordpress.com/