Dad borrowed my iphone (that I just use at home - no sim) and now I receive his Imessages, how do i unattatch his number to my applie ID??

Dad borrowed my Iphone a few weeks ago as his was being put in for repair, so he used my spare iphone to which I had an apple ID originally (i think). Now that his sim card is back in his own phone, he recieves all his normal texts, but doesn't receieve any immesages, they all go to my iphone through our wifi. How do i unattatch his number from my phone or Icloud, I am struggling a lot here... Thanks!