Dog suddenly having pungent farts

[ZilphasHatpin]

Sorry, not a nice topic, I know.

6yo dog. Never had a problem with smelly farts. I honestly believed he didn't do them! all of a sudden in the past week or so he has been making the kitchen a toxic hazard! No change in diet at all. He doesn't get any scraps at all or any access to food other than his breakfast and dinner. He has been drinking as normal. He has been on the Iams for weight management for a couple of years and before that he was on butchers. No fart problem with either food until this week. Am going to take him to the vet in the morning. Does anyone have any idea what it might be?