Under Universal Design for Learning, the same accommodation can be available to the whole class. Now if UDL isn't working for the student who needs it, then that's a sign that it's not the appropriate accommodation. Request a meeting so the 504 can be tweaked. If the 504 isn't being followed, that's a Civil Rights issue and filing a complaint with OCR is the next step if going up the chain of command isn't working.
Ask for data on how often “frequent check-in to assess for on task behavior” are finding your child off task. Your child might need a less distracting environment or a different sort of help & not more check-ins.