Sadly being diagnosed at 5 isn't very uncommon now a days. We have a few moms in our support group who's children were diagnosed at 18 months and 2 years old. My son was 7 when he was diagnosed.
I suggest you have his dr run his labs to see if anything is out of the normal range. Sed rate, albumin, hemoglibin, hematocrit, etc. Also ask them to do a stool sample. You are your only childs advocate! If you dr resists to run any tests, find one that will!
Best of luck.