OK - some of this may seem obvious but I am trying to be thorough. You probably do a lot of it already I don't mean to patronize.
Do you and your partner agree on what the message is? It doesn't matter if you have different views but be clear he knows it is what you think.
It won't be one CONVERSATION.
Equip him with critical skills. Advert in the paper "do you think that picture is real?" "Is that what that women really looks like?" "Do you think she wants to buy a car?" "Why do you think she is sitting on the car in a bikini?"
Ask him questions and nudge him, avoid telling.
When you shop "why do you think I won't buy those eggs/clothes etc?' " do you think I am right to consider the people who make these things?"
When talking about school "what do other boys say about. ####?" "When they say that how do you feel" " would a girl have a different view to you?"
As he grows up his answers will be more aware and you can question him further. Listen and respond to his answers it might get uncomfortable.
Does he know you check his browser? Have you discussed this with him. You and your partner need to have a conversation now about how long you will continue to do this. There comes a point when he is entitled to privacy.
He will see porn. It may not be violent horrid porn (on the surface) avoid telling him he is watching rape because it may well not be (controversial I know - not the point of this thread) or at least that is not what he will think. If you insist you are right he may stop discussing it. Most boys start with lesbian porn ( entirely anecdotal) so may NOT be seeing piv rape.
Respect his views even if you don't agree. He may go through a phase and then return to what you hope he will do.
You may need to do a little research if you are talking about something you know nothing about he will work it out and your opinion will have less credibility.
Push through the embarrassment your and his fathers input are absolutely crucial. It is vital you have this conversation and even if he chooses to use porn later in life it is significantly less likely to have a negative impact if he is aware of the surrounding issues.
Again I am sure you already do lots of this and I don't mean to patronise, the key is listening. It's scary how many parents don't.