My child simply won’t use the bathroom

So he’s obviously just like 2 days in, but we had this issue in pre-k as well for a bit. His teacher messaged me today, and tbh I knew in the back of my mind it would come. She said he’s holding it, but otherwise is thriving and wants to see what she can do (not that she can do much). He’s also now refused to use the toilet since coming home.

I already have a ped appointment tomorrow for my other child, so I’m not sure if it’s time to bring up another appt for an OT referral/IEP.

Has anyone else been through this? He is so bright and this is the last thing I want to hold him back. All he’ll ever say is “he’s scared of the big toilets”. We’ve practiced in public settings, our house of course, friend’s homes…like I just don’t know.

He does wear a diaper at night, and that’s obviously when he releases everything, so right now we plan to just remove it this weekend and go through the motions. I’m just not sure how to not make this traumatic for him at this point.