but what about personal messaging?
Because me and my boyfriend are trying to keep our relationship a secret for now and my brother is a constant facebooker.
no personal messaging. You need actual internet for that on your phone/mobile device, or Wi-fi if you have a smartphone
and you can make your profile private from everyone... you can also text him. How would your brother know if you have a relationship? You could go on the computer, talk with your boyfriend, and then logoff- he can never tell.
To sum it up, there's no way he can tell unless he is on your profile, or you have a relationship status changed or something really obvious. If it's personal messaging, that can be done on the computer- which I assume you are typing from- and he won't know lol