Severe pain after one month of hemorrhoidectomy?

[axbxc]

My dad had piles since 9 years (I know that's a lot of time). He had 4 nodes. Recently he had hemorrhoidectomy, which according to doctor went well, but since then it has been over one month and my dad experiences severe pain with bowel movements alond with few drops of blood. He says there is a swelling in his anal canal at the place where surgical stitches are made. This swelling narrows the canal thus requiring him to force bowel movements (thus pain and blood). He has difficulty in sitting straight and walking. He has been taking laxatives, fibres, medicines applying ointments etc as directed by surgeon, but it has been over a month and not much improvement is visible. What might be the possible reasons for this and how to treat it?