I encountered him most often in the forest and library. He can show up in the training grounds too. Not sure if it's the only place he spawns at the time, but while he was avoiding the player I found him in the forest. During his dialogues, pick the answers that are most straightforward and supportive - he doesn't respond well to sarcasm. And make sure you level intelligence (or was it good student?) as early as possible so you can help him study when prompted. I also always selected his icon immediately when it was available, when we had made plans.
I hope that helps, I never got his bad ending so I'm not sure which scenes are the trickier ones to get. ^^; I absolutely adore Simon. He is such a sweetheart, and it's a joy to brighten him up. The writing in his route is so heartfelt - maybe don't read on until you get the good end because idk if you've seen this scene - but I really liked the descriptions of his feelings & mannerisms and that bit about an underlying plea for patience in his words really tugged at my heartstrings . Lovable dork!!