He is most likely being a decent, responsible Dad, that is all. Protecting his kids from additional hurt as best he can and spending the maximum amount of time with them with no distractions.
Divorce is hard on the kids, everyone knows that. Hard on the parents too. But kids getting attached to Mom or Dad’s new partners, then it not working out, is worse in a lot of ways. Someone else walking out on them is really not a good feeling, especially if they are not that old. One that a good parent tries not to let happen.
When he does finally introduce you to his kids, you know he means business. And, when he does, remember. You are not Mom to them. You may make that grade eventually, but it takes time and trust. Time happens, trust you have to earn.