I have read of a few people complaining about bad gas on high protein diets (Acana and Orijen are high in protein compared to other foods). Some dogs can not handle the high protein.
My dog is on Orijen as well but he has no gas or diarrhea and digests it well.
You might want to consider going to a completely different brand to see if it helps his digestion and gas.
Check
http://www.dogfoodanalysis.com/ and shop for some other brands that might fit your requirements. Give him a few months on a different food to see if it helps, if not then you can just switch foods back. You might want to talk to the vet about it the next time you see them as well.