Corgis tend to put on weight easily, they are herding dogs and need to be active. I had to walk the one I had for 3 to 5 miles a day to keep her trim.
Feed measured amounts twice a day, I would not free feed. See tips in General Guidelines http://www.homeovet.net/dynamic/php/downloads/dog-c8470f2c75dbe4b683205c3919ee2310/dog_diet_complete.pdf
Make sure he is drinking adequate water, maybe add a splash to her food. I like Nutrisca Salmon and Chickpea dry food as a base, but there are other good brands. Check Chewy.com for prices and delivery.
If they have an adequate diet, they don’t need supplements, unless your vet advises differently. I do add a fish oil capsule for dry skin.
I have found this site to be informative. http://skeptvet.com/Blog/category/nutrition/
