I don't know what's causing the problem but I don't see how jumping or otherwise beating up the bike would do it.
1/2 inch slack would be way too tight on my HSUN, so maybe it's the same on a Zong. Try adjusting it with your son on the bike and the suspension compressed. You may also have a lubricant on the axle bolt that allows it to move.
Personally in 30 years or riding and wrenching that's a new one on me.
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