No Idea, it is annoying that you are compared to pro players with that mastery score. You need to be an challenger to get s.
No you don't have to be challenger. It's his cs. In both games his cs is below 5 a minute, which is very low for a solo laner to have. When both of his games his jungler outfarmed him, a top lane, which top lane is about the easiest lane to farm in, that's a perfect reason why he didn't get an S rank.
Your CS is low, that gets factored in, also apparently how much you ward also plays a part.