99 XR70 rear spokes

My boys 99 XR70 keeps breaking spokes. It's all stock but it is needing a new chain and sprocket. Do you think him using the shifter like a clutch is putting alot of shock to the spokes ? He lifts the shifter up till the clutch disengages, revs it a little then dumps it. He said he wants a kx65 or rm 65, but he's a little too short yet.

I checked his chain and it wasn't perfectly straight. Bet that would cause them to break even more having that pull to the side a little.