I wonder why they want a separate 504 and IEP? Usually this should be handled in an IEP.
In any case, a few ideas:
Clear, written instructions on all in-class assignments, homework assignments.
Assignments adjusted for timing and content, e.g., don't make him do repetitive work if he can show mastery.
Breaking larger assignments into smaller tasks.
Calming area and breaks offered when he is visibly stressed
Pre-alerts to changes.
Positive reinforcement.
Bypass peer sharing/editing (option to share with staff instead).
Option to dictate / Access to speech to text software.
