Local swelling after cervical spine (neck) fusion is a common occurrence. But this usually happens soon after the operation and is easily treated. A delayed reaction strong enough to send your father to the hospital may be something altogether different.
Did he have a bone graft using donated bone tissue? Perhaps he's having a reaction to that. Did they use a bone graft substitute such as rhBMP-2? Reports of problems using this product in the cervical spine have been published.
Treatment may include drainage of the fluid (if there is any to drain). Many times the swelling is all through the muscles and soft tissues. There isn't a pocket of fluid in one spot that can be drained. A CT scan will help show where the swelling located exactly.
Drugs such as steroids to reduce swelling may be given. Ice and keeping the head and neck up away from a flat position are advised. With severe breathing problems, the patient may have to have a breathing tube put in place.
With careful observation and treatment, the patient often goes home without further problems.