how to know if ur a rebound

i ended up finding a coworker from a previous job on tinder a couple weeks ago after my last situationship ended. i was super into him while we worked together but he had a gf who he is obviously no longer with. they were long distance for 2 yrs and when i asked about what had happened the gist was he told me she became very cold and distant towards the end and he was the one who broke up w her, which happened at the end of august i think

we've seen each other almost every day since the first date a little over a week ago and on the days we haven't seen each other we've talked on the phone for hours. i really really like him, he's extremely sweet and generous and asked to "talk about us" very soon, and i wanted to be cautious but given how often we've seen each other and how mutual the feelings appear to be i went along and now we're capital D dating and he's telling his friends about me and everything. i did ask if he thought he could be rebounding and he said no and that he had already mourned the last relationship before it ended. i just took his word for it bc i wasnt totally over my last thing when i started seeing him but i couldn't care less now and that changed very quickly. however now i'm worried again, just bc his ex comes up in conversation... like he's not talking about her as a person or them as a couple per se but for example i asked what the last thing that made him cry irl was and he said when he broke up with his ex. oh also he said they were still kind of talking when we matched but that stopped, and they still follow each other on his art instagram acc/are fb friends 😬

am i stupid?