Callus build up on the foot is due to abnormal pressure and friction as you stand and walk. It is important that the callus not get to thick or the skin under the callus can break down and cause an ulceration. It is not uncommon for me, when treating a diabetic patient with calluses on the feet, to trim a callus and find an ulcer under the callus. If the callus is not trimmed, then the infection can progress into the bone or deep into the foot. Good nutrition and vitamin supplements will help with healing but the most important issue is adequate blood flow. If you have bad circulation ask your doctor about hyperbaric oxygen treatment.
You should also discuss with your doctor about obtaining a diabetic shoe and molded insole to protect your foot.