Mario's birth and defects (pp. 314–315)
That he and his late father had been, no pun intended, inseparable. […] basically being the way any son would be whose dad let him into his heart’s final and best-loved love, lurching gamely but not pathetically to keep up with the tall stooped increasingly bats man’s slow patient two-meter stride through airports, train stations, carrying the lenses, inclined ever forward but in no way resembling any kind of leashed pet.
Context
The scene's quiet thesis, and the theme's: Himself's best fathering went to the one son who could never hold a racquet. Mario — who can't grip a stick or stand unaided — is the child the father let all the way into "his heart's final and best-loved love," carrying the lenses through airports for the last three years of Himself's life, sleeping in the same motel rooms, the honorary production-assistant. The father who is mute and distant with his tennis-prodigy sons is easy and inseparable with the one the sport had no use for, as though the pressure of the male line lifted exactly where the body couldn't carry it. The narrator has to specify "in no way resembling any kind of leashed pet" — has to defend the closeness against the reading of it as keeping — because being genuinely loved by this father was rare enough to look like something else.