Potential root causes of sleep issues in toddler I should be looking into?

My almost 2 year old is still waking up at night. He’s “sleep trained.” He understands it’s time to sleep, he can put himself to sleep in his crib, he’s never co slept. I really don’t understand why he’s still waking up crying at night, it’s usually sometime between 10-2 that he will wake at least once or multiple times.

Our ped did labs and ferritin was low and that’s been their only suggestion is iron supplement but I haven’t seen any improvement in his sleep thus far. I just don’t know what else to look into—I feel like something is causing him to wake up. He naps well during the day, is on a consistent regular schedule with age appropriate wake windows.

If anyone has any suggestions of testing or other things I should be looking into or asking his doctor about please comment!