Before getting into anything too serious, be sure that you know his stance on marrying again.
If he doesn’t want to get married again and you do, there’s a huge conflict here.
You’ll need to be extremely mindful of the parenting situation as well as the time that he has to spend with his children.
You’ll also need to figure out where you fit into the picture, as it’s likely that you may one day meet his children.
The two may still be in contact, especially if kids are involved.