I wouldn't worry about it, if his ribs are not real visible his weight is probably fine. He will continue growing for another couple of years. No sense in wording about things beyond your control. My boy Zekje was the runt of the litter, tiny compared to the others. By the time he was 18! Months old he was one of the biggest.
Here he is at 12 weeks.
Here he is now. Normally about 88 lbs but I have him down around 84 lbs now since we are getting ready for our trial, he has slimmed down, it helps with his speed and clearing the wall and jump.
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