Since his last vaccinations my 4 yr old son has had 1 swollen gland in his neck for 8 weeks now. it feels exactly like a grape. likelihood of cancer?

Unlikely: What you describe is likely a lymph node. They are more noticeable in the neck where they are not buried in fat or muscle. They are usually pea sized at rest, rubbery & move a little when pressed. They enlarge to 1-2 cm when active at filtering germs & debris from body serum. I would have his doc evaluate it. Most are insignificant.