Sodium helps retain water in the body, less salt in food might help a little. My mother is diabetic and also has swelling, hers is due to water retention, something not uncommon in diabetics. Most diabetics are allowed only a small amount of salt in their diets....u might want to get him checked for it.
My dad also has the se problem he retains water and swells up a lot but my dad also has conjested heart failer and kidney failer and that causes a lot of the swelling plus he has diabetes witch is another reason he swells up ur father need to be checked right away. My father almost died from this it is no,joke tell his Dr they need to do a lot of tests to find out what is wrong dont take no for an answer make them do the test please this is a serious matter that can be fatel good luck hun hope it all works out for you and your father