Good Day, my best wishes to your dad and his family members who are fighting this terrible disease.
Yes, it can spread and he should be scanned regularly to ensure that it has not. My wife was diagnosed with Stage IV that had spread to her liver but we caught it way early and she is now in remission.
If the cancer had spread into lymph nodes than it may have spread to other spots. We have used CT scans and Pet Scan to detect where it was. We have also used Endoscopies to get biopsies from the affected areas.
I am hopeful that you doctors are on top of this and wish him the best. Please make sure they are scanning him to detect possible other sites.