Is there such thing as a Grey/Silver Labrador Retriever? Or is it just a Weimaraner?

I'm sure there is if you mix them into a mutt.

There's yellow, chocolate, and black labs, no silver.
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According to the American kennel Club, Labs only come in 3 colors: black, chocolate and yellow. I don't think Weimaraners resemble Labs much at all but that is probably what your brother is thinking of.
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I believe silver labs are actually mixes.
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10+ years of being a shelter volunteer, pet owner and lifelong animal lover

silver/grey labs are probably a cross between a black lab and a whtie lab. :]
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Yes there is, they are a mix………only ones recognized for breed standards are Black, Yellow and Chocolate

and Wein's do not look like a lab.

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This explains it better

http://www.silverlabs.com/controversy.htm
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There are over 10 gene loci that effect coat color in dogs, with 2 or more alleles (variations of the gene) at each loci.
Labs are primarily one of three colors- black,chocolate and yellow, though there are other less common color/markings.
Labs are fixed at the dominate black loci - With 2 main gene loci that effect melanin production creating either the chocolate or yellow both of which are recessive.
The silver color like that of the weimaraner adds another recessive gene in the mix. This is the gene that makes "blue"( not to be confused with blue merle) which also effects melanin production.
The combination of the chocolate and blue creates the silver color( called Isabella in many breeds), but there are many factors, mostly genetic, that can cause variation to the color.

http://homepage.usask.ca/~schmutz/dogcolors.html
http://www.labbies.com/genetics2.htm

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I wish I had a a labrador retriever.
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