he had sex with the dad
I asked myself the same thing, though I assumed it was the dad. If it wasn't, the kid would have said something to him.
I think he spoke to the kid first then later with his glasses broken so his eyesight was not so good he met up with the dad and the conversion on the past and lovemaking were all with the dad so I think they are finally going to get together and be a loving couple
It was with the dad. Take a look again at the clothes of the make out scene and the part near the end of the story where they explained in pictures (mini diagram) how the dad was observing the seme. it’s the same clothes (the dad’s wearing a dark polo shirt).
Did he have sex with the dad or the child or he didn't do anything