Has he ever experienced anything like this before? I am going to give you the link to our Diabetes site where they may be more familiar with diabetes symptoms.
The American Diabetes Association provides a fairly comprehensive list of diabetes skin complications.
Since it is spreading and there is swelling involved I would call your dad's doctor right away. You probably don't want to wait on this.
I hope he feels better soon. Let us know what happens if you can.
We hope you find this general health information helpful. Please note however, that this Q&A is meant to support not replace the professional medical advice you receive from your doctor. No information in the Answers above is intended to diagnose or treat any condition. The views expressed in the Answers above belong to the individuals who posted them and do not necessarily reflect the views of Remedy Health Media. Remedy Health Media does not review or edit content posted by our community members, but reserves the right to remove any material it deems inappropriate.