you and your son should have separate phone lines

because you are living in separate apartments.

it doesn't make sense to use the same phone line for both of you, there is no privacy. so i guess your BF has a reason to be unhappy.

however, it is your life and you are paying for the bill, so i guess you do have the right to choose the life style you want. mabye you can give back the used phone to your BF and buy a new one yourself?