Warded Ms. MacDonald/Mr. Malfoy.
I wonder if you're aware of the fact that my mother was in Slytherin, and her family has also always been sorted in Slytherin before I came along. It's the person that matters in that moment, and their choice, not the lineage. Would you not give at least some weight to the fact that even Dumbledore cannot over-write that choice? I'm sure your son, when his time comes, will find himself in a kinder, more accepting environment. It'll be up to him to deal with it as he wishes.
-- perhaps you can propagate for more acceptance of capable muggleborn children into Slytherin for diversity and an end to what you presume is a staunch bias against the House.