They don't mention it much in the story so it makes sense that you forgot, but it's the main reason he refuses to date the Seme. Since his ex left him traumatized with his stalker and obsessive behavior, he freaked out when he saw the Seme at the bar. He thought that the Seme was also stalking him. The trauma caused by his ex resurfaced and i think that's the mental illness he was referring to

i’m not sure if i forgot or it was never mentioned but what is the mental illnesses he has??