Dull ache at the side of the neck is pretty unspecific, but there may be several causes for the ache/ pain. Commonly, it could be due to wear and tear of the neck/ cervical spine, and this is called cervical spondylosis.
As your dad is persistently symptomatic, he would need to see an orthopaedic surgeon. A complete history and physical examination would have to be done, and often times further radiological investigations (X-rays, MRI) would have to be done to confirm the diagnosis.
The problem can then be treated accordingly, and is usually done via medication or physiotherapy. Very rarly is surgery indicated. Hope this has been useful.
Best regards
Dr Sean Ng