Only if you had had an ultrasound in something like your 7th week, and from that measurement of the embryo were given an estimated due date back then, can you try to make a guess now that would have enough validity to rule out your ex as too early. Your option is the DNA-test route, which would clear things up nicely. I'd do it.
But I know the sex with my ex was not past the 23rd
Well I've been looking back on Facebook and I know me and my ex were broken up by aug 23. But idk how soon to that we had sex. Me and my current didn't sleep together till atleast the end of the first week of September. Like I said I had her may 30th which would make it seem like it's my currents. But I had her so quickly for me to only be "38 weeks" makes me wonder if I was really 40 or more weeks.. That would mean it's very possible for her to be my exes. Ugh this is insane. I don't know what possible.
If you think the sex with the ex was as late as perhaps the 30th or so, it might be worth a DNA test. If you do the drugstore kind of test, send swabs from *both* guys, be there when they swab, seal the whole thing up right there and mail it yourself. If the answer comes out a surprise, you'll need to go the certified-lab route to get a more official test -- you can call the family courts for information on how to find a lab certified by the courts to test for paternity.
The problem you have is that you don't know for sure how accurate your initial due date was, or how close to term the baby was born. So only a DNA test will confirm the dad.