My son had surgery to remove a plate inserted in a broken clavicle 8 years ago. the doctor was unable to remove the plate during surgery. should the doctor have foreseen this prior to surgery?

No, no surgeon will attempt removal of hardware unless they believed it was technically feasible. It had to do with unexpected findings that made removal impossible.

Hardware removal can be surprisingly complicated and things that are not predictable are inherent to surgery. So while the surgeon may have planned for removal unforeseen circumstances can and do happen that may have made it impractical or cause more harm than good.