Check where the post inserts into the frame. There's a little gap in the frame tube there, so the post can slide in and then be tightened with the bolt. The sides of this gap should be parrallel (sp?). If the top end is closer together than the bottom, he's been riding with a too-small post. Stick a hefty screwdriver in the gap and wiggle it a bit to spread the gap. Then try the larger post.
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