Her son is making me feel so unwelcome

I prepared well for that day, but my fiance's son is making me feel so unwelcome. I did what that marriage tours had suggested on meeting one's fiance's family member, but it didn't work so good. I know I did well on her parents but the problem is the kid. It seems like he doesn't like me. I tried my best to look good in his front but to no avail--he just sneered at me. Wedding is happening in two weeks now and I need to win that kid's heart first. What should I do?