Yes, you can be with him alone. But only when you meet the requirements…
Well, the only 1:1 I got was when he wasn't drunk and I disclosed Julian's plot, then Julian leaves and I stay with Ron, but then nothing really works, if I choose "caress" my score first rises, but then takes a dive cause Ron starts to talk about what I did with the couple. All other choices first decrease your points, BUT they actually avoid that topic and the loss of point is ultimately less, which I found rather incoherent. Then Ron goes to sleep and the game ends.
My relation score with Ron at the beginning of the scene is 4.75.The only other variation that I might be missing is if I never went to play with the professor and his wife, which might prompt Ron to say something else, but does it really change anything to the outcome of the scene ? (cause I would have to restart everything)