Hi Mariah,
He may still be getting upset even though you both had a “closure” conversation because every time he hears about you or sees you, he gets a reminder of how complicated the relationship between the both of you was for years.
He is also reminded of how it all ultimately ended in you turning him down when he was most likely finally ready to get into a relationship with you.
Though he may currently have a girlfriend, those facts still sting. They sting of failure and shattered hope.
The “closure” conversation didn’t automatically wipe out the history that he has had with you.
There is a good chance that the only reason why he got into a relationship with his current girlfriend is because he was never able to get into a relationship with you.
That means that there is most likely still some emotional baggage that he has yet to reconcile with.
He may try to make himself believe that he has moved on and his new girlfriend may have been a welcome distraction to that effect.
However, whenever your presence becomes a factor in his life again, all that history comes back and he finds it hard to cope, hence why he gets upset.