Does he have OCD, then? If so, I'd seriously look into cognitive behavioral therapy--a non drug approach to treating OCD that is quite effective.
It is my opinion that the longer OCD goes on the more the brain patterns change and become entrenched. Cognitive behavioral therapy changes those brain patterns and can break that cycle--and is research based and widely accepted as an OCD treatment that is equal or nearly equal to drug approaches for many people.
As far as herbal or natural approaches--the same medications anyway that work for other anxiety disorders don't work for OCD. It is a different type of anxiety disorder altogether. So while I don't know of herbal treatments I would suspect they would be different than those for other anxiety issues. The medications that do help are anti-depressants like paxil called SSRI's so you could approach your research looking for alternatives to SSRI's (like St. John's Wart though of course anytime you're dealing with herbals you are also dealing with a medical substance by their very nature).
You can search online and I know there are books on amazon about OCD in children and cognitive behavioral approaches though I've not read any of them personally. It would depend on where you are if you can find a competent practioner (bigger areas yes). There is an OCD foundation
http://www.ocfoundation.org/ which is very active and I know they could help point you in the right directions.
GAD I know less about but honestly if a person has OCD of course they are anxious all the time. It is a stinky disease when it isn't managed/treated. Kava Kava would be a pretty standard herb for anxiety.
I've said all this and make sure the symptoms you are seeing are really OCD. If they are what I imagine them to be in order to be confused with a spectrum condition I think you need a professional to determine for sure. A spectrum individual could at least outwardly look the same with an adherance to routine or order for example. But the motivation behind it would be different.
OCD is actually a brain difference (can be seen on scans of the brain). Great diet is always a good thing of course but I wouldn't expect it to impact OCD. Like most brain differences there are also good things about it....just help him learn to manage and minimize the bads. As an interesting to me aside, there are some who believe that autism is actually on a broad OCD spectrum. Not at all sure I agree (I don't even necessarily think all we now call autism is actually the same) but I see the potential over-lap.