I got far on it accidentally. After seeing him in the gym and complimenting his body, the game seemed to turn on his path. You have some real and dream sex sequences if you keep finding him and eventually he appears in his room again, though I am not sure if the final sequence is a dream. I'd recommend just saving before any dialogue with him and trying the different options. If you end up going out of town with him and he gets drunk, you're on the right track.
Viewing post in Other routes
I wish I knew! I was trying to follow Thomas' route and even went on the date with him, but then it switched to David. I would say just follow Thomas by sleeping in and going to dinner with him but also keep checking the gym for David. Eventually he summons you to go to town with him. It's frustrating, I know...