We've only had two lethal white bubs (thankfully...) and Darla their mum, their siblings, older 'cousins', three aunties of their four aunties were great with them.
I'd keep an eye on how they interact with him. If it was just mum, I wouldn't worry too much about it as she most likely knows that he isn't 100% right..but for even his siblings to not be very nice to him doesn't sound normal from my (little) experience...as Dot & Deka (Dede & Delta's sisters) were amazing with them and Dot would always be cuddling up next to Delta. They loved their sisters.
Has he seen a vet?
If so, what did the vet say about his molars?
As for his weight. He was born a big boy...- close weight to Zeke, at birth he weighed 120 grams, 3 days old he weighed 127 grams.
At 4 weeks Dede is just in the 160's..even though she born a normal girl at 95 grams. Three days she weighed 83 grams, six days she weighed 96 grams.
Delta was born at 109 grams, Three days she weighed at 102 grams, six days she weighed 123 grams.