He apparently suffered a minor stroke. From his son, Alex:

Jerry had a small stroke. He is recovering well at a local hospital. Prognosis is good, though they’re running more tests and he’s expected to stay at least another day or two.

“He felt well enough to call Mom [Mrs. Pournelle] from the hospital.

“Thank you for your thoughts and prayers. More updates when we have them.

I saw him a few weeks ago, and he seemed to be doing as well as a man his age who has recovered from a brain tumor could be expected to. Best wishes for a rapid recovery.