We have had good results with Alliance Pediatrics although our situation was not as complex as yours. You could ask if they have worked with patients similar to your DC.

https://alliancepediatrics.com/

Reach out to the colorectal clinic at Children's National. We had tried EVERYTHING, including local ABA coaching, and they were very helpful with helping rule out or investigate some things we hadn't considered, including PT specifically for toileting. There is also this ABA toileting program at Emory, which isn't local, but could be an option.

https://www.marcus.org/care-and-services/complex-behavior-support-program/toilet-training-program

Hi,
Does anyone have direct experience for getting services to help an older child (12yo) with autism who refuses to use the toilet. The issue is not the noise in the bathroom, or sitting on the toilet, or not knowing the steps. He is willing to do every part of the toileting routine other than voiding in the toilet. Any information about specific ABA or OT or psychologist who has helped would be so appreciated. We are in Fairfax but are willing to go anywhere in the area. Thank you!