Ah, so that's what it was. I love pianos, and I always have to go up to one and find out which model it is, LOL...
I agree, that thing is a monster, and there is no way a 5-year-old could have pushed it with that momentum even under an adrenaline rush. Hell, I'm 110lbs and I have trouble pushing a concert grand across a hardwood floor all by myself. I lean against it with all my weight and....it budges. With the weight of a 5-year-old, he simply could not have produced enough leverage against it. 
Unless he's an outrageously dense kid and Lois and Richard have to have superstrength to be able to pick him up. 
 
Lifting the piano would require only brute strength, but it was clear that Jason pushed it, and the force of a push depends on your weight. Or more specifically, how much you can grip the ground. Was Jason wearing soccer cleats? 
