The fact that your son was treated for OCD for longer than 6 months will be a disqualification, no matter when it occurred. If I'm reading your statement correctly your son has not undergone any therapy for at least the past 4 years, and has never taken any medication. If that is true then I think he should have a very favorable shot at receiving a waiver.
I am basing my statements on what you have stated. DoDMERB will request all medical and treatment records (the therapists records) concerning the OCD. It could be that once DoDMERB reviews the records, they may feel that he doesn't meet any of the disqualification criteria.
When your son fills out the medical history questionnaire I would have him explain in as much detail as he can what happened in his own words. Also be prepared to get the therapists records. I would also let your son know that he might be disqualified, but that a waiver is possible.
If you have any other questions through this process, please feel free to ask!