Dabney Coleman's Character

In the beginning of the film, there's the older gentleman, J.B. Parker who owns the thoroughbred, plays poker with Coburn, and gets pissed that the train would be leaving without his horse because Hackman was late. Then he just disappears for the rest of the movie and Dabney Coleman shows up as his son, makes the race bet with Coburn, and fires Hackman for showing up late.

Did something happen to the actor playing the father requiring him to pull out? It just seems odd to have two characters being father and son, but not appearing together and fulfilling the role of a single character.