As his son, Ken, wheels him up the concrete ramp and through the doors, Lindsey's grin says it all.
Lindsey is frowning because he just pushed Ken down a flight of stairs. 
mnli hypothesis: Lindsey is frowning because he just pushed Ken down a flight of stairs.  premise: As his son, Ken, wheels him up the concrete ramp and through the doors, Lindsey's grin says it all.