Ah, 6 is a bit young for that fantasy to be spoiled. It may have been his mum that told him, but I found it best to wait until the child found out for themselves. I would be a bit disappointed, yes.
My kids - 2 of my own, 2 stepkids, all went up a year in age. When the first found out santa wasn't real, he kept up the pretence for the other 3, and so on and so on.
I'm sure you will still make it special for him anyway, and at 6 - he probably still believes santa is real & thinks his mum is soooo totally wrong!