Stranger things have happened. I just don't see how a "fall" could be deduced from that. Why not he "was pushed," especially if they thought his son was around or somehow knew something? Just funny, to me, that it all changed when the defense attorneys got involved.
They probably found the site of the fall, usually a table corner, fireplace fender, or similar sharp-cornered object, with blood and hair adhering. The angle and mechanics of the injury can give a pretty good idea of whether he was pushed with any kind of force. But really the only way to get that information pinned down is through interviews. The son I'm sure has given a statement, probably multiple statements as the investigation progressed. And - police procedural novels and CSI notwithstanding - most criminals aren't very good at keeping a consistent story through multiple statements, especially in the case of homicide, which usually shakes up even experienced investigators (let alone the participant).
The amateur murderer often forgets that the police are way ahead of him in terms of their experience in the field . . . .