Please don't go against your brother's wishes. If you tell your mum, chances are he'll never confide in you again, and may well need to some time in the future. It's his news to tell, and hopefully there'll be no need for him to say anything anyway, so you could be worrying your mum for no reason at all.
I'm on the receiving end of a health secret of my younger brother at the moment, and though I do think he should tell our mum, it certainly isn't my place to make that decision. Just be patient and wait for the results to come through before making any decisions?