Doesn't sound to me like he is growing too quickly.

Siren weighed 12.3 at 8 weeks.
20.3 at 10 weeks and 5 days, and 23 1/2 at exactly 12weeks.
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Siren vom Banach ( Sable female GSD.){3-20-08}

Wrangler (Blue Merle male ACD/Aussie mix.){9-29-99 to 8-29-11}