Diversion is a term that is used to describe placement in an informal program in which you pay a fee, stay out of trouble and charges are dismissed.
If you mean expungement ... he would be eligible to expunge from his record after 5 years passes from the time that he completes sentencing.
I don't practice in WA. If, like the other counsel states, you mean expungement of 16 year old DV cases, then probably yes. Depends on whether they're felonies or misdemeanors.