just to preface this, i have 2 older children who I've not had this issue with. My DS who is 7 has erm quite the interest in women always has done, he always liked women in music videos with large bottoms, and, in catalogues always fixated on pages with underwear etc, i used to think it was a bit strange but ultimately harmless and was just some kids develop earlier. however last week i found hed tried to type bikinis into youtube. everything is on restricted so thats something already in place, i had a small conversation but whilst trying not to shame him, im a bit worried he might become a bit pervy and i feel awful feeling that about my own child.
You mention two older children, presumably teens, although you haven't said, but they will of course have more of an interest in the subject of girls, which 7YO will pick up on. The best way to get your 7YO to respect woman should start with ensuring he gets to play with girls his own age, and you can if neccessary tell him what is acceptable/appropriate and what would not be. He seems like a happy healthy and inquisitive boy, and if he wants to see a bikini, show him one.