The simple solution is to feed your dog less of everything, cut the treats, titbits and leftovers, and start exercising more. To stop him feeling hungry, you could bulk up his meals with fibre, such as cooked pumpkin, carrots or unprocessed bran. Check food labels for the right amount of food for his target weight - but remember these are usually a guide for when this food is given exclusively.
To avoid the temptation to overdo the titbits, try putting all of your dog's daily food allocation (including treats) in one container. This will deplete gradually throughout the day, and when it's empty - no more feeding Fido again until tomorrow! Sometimes changing to a smaller feed bowl can help too. A smaller serving looks a lot bigger in a little bowl!