Hello,
At 8 months if a child is not able to hold his head properly and sit unsupported then it is a delay in his development. With this limited information it is not possible to come to a conclusion. It will be advisable to consult a neurologist who can examine and find out cause of the delay. He may have to undergo a series of tests and examinations. If required doctor at your place may refer him to higher center for further tests.
Good luck.

Hi welcome to HCMForum, He is not sitting properly crying all the time doesnot mean that he had problem in the neck, at this age any baby can't sit properly with out support. the cry at this age suggests stomachpain or ear pain besides ear pain also cause bending of neck to the side of the pain. So there is chance of thinking that there is neck problem.
I advise you to consult a pediatrition for diagnosis and treatment. you may have to get his stools tested for reducing substances along with other routine tests. if no relief consult anENT surgeon and lastly to an orthopedic surgeon.
Besides i advise you some diet recomendations as To the child try to change the type and concentration of the milk and weining food.
For mother avoid spices chillies junk foods and oily foods if breast feeding. Thnks for calling.