My father, who has advanced Parkinson's disease and in his eighties, has fallen and struck his head. What do I look for?

This isn’t a good time to go to the ER—his immune system is compromised due to chemotherapy.

He seems lucid enough; albeit slow to respond but this is fairly normal for him. He has a mild gash that stopped bleeding fairly quickly, not much worse than when he used to hit his head on the attic rafters.

I know my fellow jellies cannot give official medical advice, but this isn’t a good time to go to the ER. Most medical websites are about after-effects of falls. not the immediate. What should I look for?