Introducing hearing five year old to deaf partner

My son and I are hearing. I am in a serious relationship with my deaf girlfriend. My son lives with his father in another state mostly but visits me a lot. My girlfriend lives far away in the opposite direction. As such, visits have been all over the place, and my son and girlfriend have yet to meet face to face. However, we intend to live together at some point. For now, I skype my girlfriend when my son is at my house, and he gets very overjoyed to see her. He was shy at first, but now begs me to Skype her. He is only five but is learning to type a bit and will send little messages. He constantly asks about her, and all that is great. She is also excited to get to know him. My question is, how do I get him to understand that she cannot hear him? He talks to her and I tell him over and over but he does not get it LOL he is young and innocent and believes she can hear him no matter what I say. Any tips? Also I do not know much sign, neither does she, she is late deafened. But we know basics and finger spelling. Would like to introduce this to him, but a challenge to is how little real time I have with him, just a weekend here and there. Anyway, thanks for any advice from anyone who has done this

Maybe you can take him to a quiet room or very quiet church where there is not a drop of the nail on the floor. He might get the idea why your girlfriend is deaf. Maybe you can go the hearing society where the counselor(s) will give him special ear muffs which has no sounds at all. He will learn what silence is like. I hope that will help him.