I'd just like to meet her to clear the air from the past
Oh no no no!
You can clear the air by just letting them get on with their relationship. letting her build a connection with your DS as well, and staying out of it. You will naturally cross paths at some point, but making a big deal of it now and then (it comes across as) withholding contact for DS until he agrees to introduce you, can only cause trouble.
If he introduces her to you before his DS he’s essentially saying that you’re more important in this situation than DS, which won’t cause harmonious relationships between you and new DP, it will make her resentful of you.
I’d put money on there being an overlap when you were together or recently split and trying to reconcile etc when he was also with her. That’s why she was off with you and that’s why he doesn’t want your to meet up now.
He agreed to it previously as he was probably wanting to keep involved in your life, not for you to have a say in his!
You don’t have a legal leg to stand on if you try and force this. You may argue that you have a moral one, but even then, that only stands if you push the “we agreed” angle. In reality there’s nothing to be gained from you meeting her before your son does. i think it’s fine to stay you want to meet her at some point so you know who your son is spending time with, but tbh that could be any time.
If my DP had insisted I meet his ex before his DCs I’d have put off meeting any of them and probably run a mile instead!