No, based on what you write, do not seek legal action unless both of the following are true:1) Your son suffers some serious and longlasting (or permanent) harm or impairment, or you incure tens of thousands of dollars of out-of-pocket (not paid by insurance) medical costs. Medical malpractice or medicine product liability cases are very expensive: unless there is life changing injury or large medical costs at stake, they are not worthwhile.2) There is medical evidence--for exampe, the opinion of a relevant doctor (someone whose speciality and experience is relevant), or peer-reviewed papers in well-regarded medical journals--linking an expired vaccine to the illness your son suffered. Without medical evidence to show causation, you can't prove your case.