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.