Another iPhone is using my number? - 1

I have just installed ios7 and so did my dad, and i'm using his apple id but since i installed it he can read all the i messages that i receive when i go on my dads phone and go onto settings and messages then press send a d receive there's 3 options his address his number and my n ember now how do i get rid of that and also his number is there but it is grey and not ticked which means he can't receive imesseges onto his number! Is that fixable and how?

Its possible so try resetting it.