Hello, There can be various reasons for his bad smelly farts. There are plenty of medicines including METFORMIN which can result in smelly farts. The other important reson could be consumption of foods containing more up absorbable carbohydrates like beans,broccoli,cabbage,prunes,raisins,lentils,pulses,apple,brussels sprouts etc which remain in colon for a long time resulting in more decomposition of them. Decomposition results in production of sulphur gas which can cause smelly farts. Some sugar free foods containing fructose or sorbitol can also result in bad smell. Sugar free chocolates and chewing gums can also result in it. Some times a person may have lactose intolerance which can result in bad smell. You have to make a list of the foods and medicines being taken and then you can find and eliminate the offending source. Take plenty of water and avoid constipation. I hope it helps Thanks