He didn't notice it at first. He was 13 at the time, what could he have noticed? But it was there. His father had lost a stride in his step. He wasn't as graceful, wasn't as smooth, wasn't as aware. There was always something distracting, something that took his father's attention away from what was going on right now. It was just a glimmer, an extra strain in his father's features, unseen by an untrained eye or, in this case, an eye trained only to a certain level.
He was only 13, what could he have seen? What was he supposed to see?
Hindsight is always 20/20. Now into his adult years, he can see it clearly. The staggered walking. The labored breathing at times. The pauses. The winces. The often-too-long leanings on counters. Clues, evidence, pieces of a puzzle that were all there, spread out for him. A Higher Deity must have said, Take your pick, Shawn, all paths lead to the same result.
He feels guilty for not seeing it before.