Is it ok to ignore him?

I am 20 and my ex step mother's nephew that is about 14-15 that is a little mentally handicapped keeps messaging me on fb. I have only seen him a handful of times and never really spoken to him. I honestly find it kind of odd and I'm not sure I want to talk to him. It's ok that I ignore him right, or is it rude??

*** For the jerk in the comments saying I am not talking to him simply because he is handicapped STFU! That is NOT why I don't want to talk to him. I have a sister that if she had lived would have been very handicap. I have many family members and friends that are handicapped. I LOVE handicapped people. That has nothing to do with it. I only put that in post because I feel so rude for ignoring someone like that. I have said hello to him, I just don't feel the need to talk to a 14-15 year old that I will never see again and that is no longer family. We are almost complete strangers! I would feel wierd even if he wasn't handicap! So before you go around judging and being rude and accusing like that, get all the facts first. And I don't see handicap people as different from me. I see them as beautiful, wonderful, blessed individuals with a beautiful purpose in this life!