It could simply be severed nerves from all his cancer surgeries. It doesn't necessarily have to be a stroke.
If he did have a mild stroke, one severe enough to affect the side of his face like that, ...it would also impact his speech and I haven't seen or heard anything like that. Unlike other muscles, the facial muscles are rarely exercised and aren't attached to the bone. Over time they will naturally droop, and if they been cut apart repeatedly, the areas affected will droop more apparently. It's probably the result of numerous facial surgeries.
