Preserved right to buy

Hi I’m new to the forum but I need some help please my dad has lived in his house since 1971 his grandson has been living with him for just over a year, the council he was with when he took the tenancy was sunbury urban council and there has been a few more after them but now it’s a housing association,when I asked my dads housing officer for a copy of his tenancy agreement she sent it through by email and it says on the tenancy 1996 yet on his online rent account it says 1991 I have asked why it says those years and no one has contacted me back then yesterday I sent a email to the complaints department and they have said that they would look into it any suggestions on what else I can do thanks mary