Focal Cortical dysplasia-MRI showing almost gone?

My 10 year old son starting having grand mal seizures (during his sleep) back in January. He was very healthy up to that point. MRI finding stated there was a right parietal cortex lesion measuring 15 mm. Findings suggested cortical dysplasia, slow growing tumor or possible malignancy. His seizures would leave him partially paralyzed for a few hours after on his left side. He could not move his arm, face dropped, foot would drag. They said this was consistant with where the "abnormal area" was. He was started on 1000 mg of Keppra every 12 hours. No seizures since. He had a follow up MRI every 3 months since. The area remained stable at 15 mm and last MRI stated they were pretty certain it was focal cortical dysplasia. He just got the results today of his most recent MRI. Now the spot is "barely visible" and measuring 1mm. The dr could not really give us an explanation. He said he has no idea what this is. He still thinks this is FCD. But the MRI report from the radiologist says they are leaning against the dysplasia because that typically does not change. So what could this be??? What could be stable at 15 mm and then be almost gone now? He is to have another follow up MRI due to the "unusual findings" but not for 6 months.