Oh geez, a grown-up man with zero emotional intelligence and twisted/toxic behaviors already, is being played by a professional psycho, to target another emotional handicapped, omg. No wonder this sh1t got so messed up (is super interesting though).
Obviously the professor already had all these elements and situations that weren't healthy inside him, that's why he fell so easily in this turmoil. I don't think Glen planned for him to start sending gifts, but he saw and knew that this guy who was in his worst moment (who probably had some issues already, which is why he got divorced), would take anything that could offer him a scape to such sour feeling in his life.
He probably waited for the professor to start acting weirdly so he could take the opportunity to make him the bad guy in front of Joseph, that way he'd be left vulnerable due to all the trauma caused by the professor and who would be the 'hero' or good person who would protect and cheer this guy up? Exactly, Glen just "protected" him in order to trap him forever.
The reason why his ex-wife asked for a divorce, it was exactly because he just wanted the perfect doll to match his preferences and that's all, so it makes sense that now that he paid attention to another person, he was willingly sending gifts over and over to start dolling him up. Yeah, he was sick from the very beginning but Glen just redirected the flame to start burning Joseph.
As I said, I feel like Glen is a psycho, and probably at the beginning, he just wanted to have fun, I just don't know when he started to get obsessed with Joseph in a different way. Or has he always been obsessed with Joseph?
Gosh, Glen is evil, I wonder what, how, and why, lol.