What does 'form for calling people at his work mean'? I would imagine that many a dw has form for this if they were expecting to talk to their dh but found he was otherwise engaged and had to leave messages with his colleagues.
It seems to me that wanting to get the divorce out of the way before introducing you to the dc is misguided thinking as that gives ample opportunity for the dc to reach the wrong conclusion, namely that he left their dm for you, whereas being open with them now may even go some way to ease the divorce process, particularly where childcare arrangements are concerned.
As it is, I find it hard to believe that his teenage dc "aren't aware really of what's going on". They've surely noticed that their df is no longer in residence and, as they only get to see him twice a week, I would imagine that they have expressed some curiousity as to what he does with his spare time.
I don't find anything unusual about a stbx wanting to know about their successor and I take the view that, if there was anything sinister about this, there's as much chance of them turning up on the doorstep after divorce as there is before the law has called time on their marriage.
If I were you I'd be encouraging him to be a lot more open with his dc and with his stbxw as secrecy in these matters can give rise to needless suspicion which, in turn, can set the scene for misunderstandings and unnecessary hostility.