If there is a shadchan between you, this is the type of question you ask ask him/her, who can find out more directly from the guy.
If not, you have to take cues from him. He already mentioned his ex and kids, so probably does not mind talking about them, and may even be wondering why you have not asked yet.
This is an important issue and should be raised earlier rather than later.
As to why he did not get married since- don’t hold that against him, it is not so easy to get remarried, and it could very well be that he needed a recovery time before he started dating again. Those years would have been time for wounds to heal, for new family dynamics to work themselves out, for self-improvement to be ready for a new marriage.