took my son to an ER for eye problem, doctor never looked in his eye said it was a sty. Next day I took him to eye doctor and they found a piece of

There was a failure to diagnose an operable condition, but the question is whether the delay in diagnosis harmed your son. Ultimately, it sounds like he would have required the surgery to remove the object anyway, unless you have some evidence to suggest that the metal migrated between the visit to the ER and the visit to the eye doctor.