Wounds that won't heal are unfortunately a very common complication of long term poorly controlled diabetes.
Given he has these wounds, he needs to take special care to protect his feet all the time. Walking barefoot and out in the rain without proper protection is not recommended. Any serious worsening of the wounds or infection could end up in amputation of his limbs.
His feet should be inspected every day and he should seek medical attention immediately if he notes any changes.
The best chance to improve the healing is to normalize his blood sugars (or as close to normal as possible).
Normal blood sugars would mean keeping his blood sugars in the range of 80 - 140 all the time, and as stable as possible. A1c ideally less than 6.0, definitely less than 7.
How to do this? Low carb diet is a very important tool. Weight loss, if necessary, and insulin (dosed in basal-bolus regime) and closely monitored. Exercise helps with blood sugar control. Will need to be done carefully given his condition.
Hope this helps you and your dad.