I would choose one food and leave him on it for a while.. couple months. Sometimes it takes longer than just a month for the dog to adjust. And switching them around a lot will cause gas. Some treats can also do it. Or too many treats in a day can. I know that I bought this 8lb bag of milkbone-like treats from Sams and all three of my dogs get AWFUL gas from them. So now I have like 7.75 pounds of them and I dont want to give them to the pups!!

Also, some dogs just more gassy than others.. lol. Our Eli is way more pungent than the other two... But Ive heard this is common for Boxers.