Why doesn't my boyfriend want our relationship on Facebook?

[Bella]

Ok, like I've said in a previously asked question my boyfriend and I have been together for 3 month, we have a close and happy relationship and have met each others families and friends. We go to the same college and walk and sit together. People have asked him if I'm his girlfriend when we've been seen together and he's not afraid to say yes so I don't think he's embarrassed of me or what not. He said when we got together let's keep it of Facebook because it's better not many people knowing and getting involved. I'm 16 and he's almost 18 so we are not children and are very mature. I have had feelings that he could be talking to other girls on Facebook and maybe that's why he doesn't want our relationship on Facebook but I really don't think he's like that. What should I do?