I have an iTunes account / library and my partner has just got an iPhone (I don't have one)

We share the same pc so, how can I set up iTunes so he can access my music library, yet if he buys anything on his iPhone, like apps, it charges his account and not mine. As he's not really computer minded, I'm thinking that if I let him use my sign-in, anything he buys / downloads directly from his phone, will be charged to my iTunes account

We don't have seperate user accounts on the pc. Just one sign-in and one iTunes (with my card details)