After a month he should be well settled on his new diet and if he has had no problems up to now it's probably not his feed that's causing the wind. If his poo is ok and this flatulence has come on suddenly it's more likely to be something else he has eaten rataher than a sensitive tummy. Has he had anything else to eat recently, chews, different treats or anything like that? He may also of picked up something whilst out and about. Cut out everything but his main food for a few days and see if it clears, keep back a handful of each meal to use in place of treats and make sure he has fresh water changed regularly.