Friend Drifting Apart

I am a shy person and don't have many friends. I have one friend who is closest to me. We used to spend lots of time together and also would spend sometime with his friends. Lately, he doesn't seem to want to spend time alone with me.  He just wants to meet when there are other friends with us. We kind of seem like strangers when there are other people. Many times he ignores me when there's more people. Now we also seem like strangers when we are alone coz we almost don't speak. I'll give an example: The other day I invite him to come to my place and he just wanted to use the internet to speak with other friends and watch TV but me who was right there he almost didn't speak. I don't know what to do anymore. I value this friendship very much but it seems we can't find much to talk about. I wonder what goes on his mind coz I can't understand why he's treating me this way. I've just been crying lately coz I don't know if I should tell him I feel sad with this situation or should I just back off and see what happens. What should I do?

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