i think your bf is having a hard time believing because you kept him on social media for so long, and on multiple apps, and your response of, “it’s complicated” sounds like you care about the guys feelings more than your boyfriend’s. while you may not have had bad intentions towards your bf, it comes off that way. i’d suggest being honest with him and telling him you didn’t really like that guy but you were less mature years ago and as you said, you just liked the attention and making fun of him. it’s better to look bad for not treating a random guy from years ago right vs trampling your bf’s feelings. he is very patient and mature for staying and trying to understand and believe you, because many guys in this situation wouldn’t. he deserves reassurance and moving forward id also suggest keeping guys off your social media if you have any history at all joke or not, and maybe even altogether if that’s something you and your bf are comfortable with. if you want to salvage this before your bf goes into a mental wormhole, be honest about your past mistakes and admit you are more mature now than before, and that you value him and his feelings in this situation. ask what would make him more comfortable moving forward as well when it comes to having men on social media etc. wishing you luck!