Suggest Cure For Soreness In The Calf Caused By A Cut

My fiance has a four year old knife wound on his calf. He was working in a catering business and walked into a knife that was stored improperly on a lower shelf with the blade out. When he went to the emergency room they made a cut on the skin near it and stitched it over the wound. Ever since then he's had a large scar where the wound was and now there is a sore in the center of the scar. We have tried using otc first aid medicine such as neosporin and bandages as well as covering it with medicine and leaving it open to the air but the sore is just getting worse. What could be causing this? Is it serious? How should we treat it?