I have had (or have) both prostate and colon cancer -- both primaries.
If Roger C's description is correct, "stomach or upper bowel", this would place his father-in-law's tumor outside the colon and beyond a colonoscopy -- at least the ones I have had which stop at the exit from the small intestine (upper bowel).
My non-medical opinion is that this problem likely has nothing to do the original prostate cancer. While recurrent prostate cancer can lie dormant for years, it nearly always shows its presence by rising PSA levels -- rising long before before there are any physical symptoms of metastasis.
Roger, let's hope for a good outcome for your FIL -- these things often turn out to be not nearly as bad as we fear.