As someone who unfortunately has a lot of experience with this sort of thing I'm going to give you a list of possibilities/things to try.
1. Get a fecal test done ASAP! Parasites and/or Giardia can cause diarreha and gas and often these are the only symptoms. If he tests positive for one of these, you will need to treat with appropriate medications.
2. If the fecal test comes back negative, then you may want to consider the possibility of a food allergy. This means you look at the protein source(s) of whatever food you are currently feeding and switch him to a different QUALITY food (check dog food advisor for help with this, I personally feed my dogs Fromms but I live in the USA so I have no idea where you are and what is best there) with a different protein source. While you are testing this food the dog gets NOTHING but that food for 6-8 weeks. If the diarreaha goes away, you now know it was a food allergy. If not, you can try another protein source. Do not just randomly give the dog table scraps to test for allergies. If you want to feed human food, you need to look up one of the many developed RAW or COOKED diets formulated to meet their needs and you will need to supplement with vitamins.
3. If 1 and 2 are not your problem, have a full blood panel run on the dog to test his vitals (you should prob do this if the diarreha has been going on for a while anyhow). You may find the problem here.
4. If the dog is becoming dehydrated, you will need to have your vet put him on iv fluids for a day and possibly give a B12 injection as well.
5. If none of these things resolve your issue, there are prescription dog foods your vet can prescribe that are made for various stomach issues. They may either be temporary or for the life of the dog depending on the circumstances and effectiveness.
If your vet is not willing to try these things, one you need a new vet, two you need to find an internal medicine specialist. Not sure why your vet wouldn't have suggested this stuff in the first place but good luck to you. Feel free to ask if you have any questions about anything I've said
Edit, just remembered a couple other things. Consider adding a probiotic powder to the dog's food (you get it from your vet) to help add good bacteria to the intestines to combat the problem. You can also give canned pumpking (pure pumpkin, not pie filling) to help bulk up the stool and the dog will probably love it