I agree with the aforementioned possibilities & considerations. Although, it is a little early for radiation necrosis onset, which usually presents 6-12 months post radiation- but it should still be part of the differential considerations.
Additionally, if a brain MRI reveals no new masses, these symptoms could be related to an early paraneoplastic leptomeningeal disease presentation. While this is uncommon, it often presents with persistent symptoms despite clean imaging. Dexamethasone to manage the symptoms & as long as the patient symptoms are under control, continuation of ICI therapy could be an appropriate course of management. These can be such tough cases, good luck with your patient and thank you for sharing. Looking forward to an update on how he is doing.