Wondering if theres a reason he's doing this

My ex left me and got with someone else almost straight away. It's only been about 3 months but the last few days he's been turning up unannounced at my house to see our child. He has him every weekend so it's not like he doesn't see him, and him turning up like this is really not like him at all. While he's here he talks to me about our past, places we used to go etc. Asks about my family, which he never does! When he's not here he'll find something to txt me about and then it will turn into "remember the time when..." I'm wondering if he's realising the grass isn't always greener on the other side, obviously he wants to see his child but a lot of this stuff happens even when our son is in bed. I understand healthy Co parenting and being civil to each other but we already were, this has kinda gone up a notch. Do you think he could be regretting his rebound relationship?

I wouldn't let him in if our son was already sleeping, but sometimes our son will fall asleep while he's here so he'll put him to bed but then stay for a chat afterwards. I wouldn't just throw him out straight away because this guy used to be my best friend and we do actually get along. I just think the break up is only now starting to hit him, because he jumped straight into another one so soon he didn't have time to 'get over' this one if you know what I mean. Now a few months on I think he's realising. Even though he ended it and I was heart broken I wouldn't take him back now, but I also want our child to have the best parents so I can't bring myself to not let him in either. I just wanna know what could be going through his head, and how he's feeling without actually asking him because I don't want him to think I want him back and don't want that convo with him