I know this is something lawyers recommend when the wife is not consenting to a divorce. Though our law (except Kandyan Law and Muslim Law) does not recognize a divorce with consent , practically divorces are obtained by consent since the other party does not object when they are consenting.
In this case the Man can be charged for bigamy. By virtue of the setting aside of ex-parte judgment his earlier marriage is legally recognized as an existing marriage, so the man had married twice . He is liable for imprisonment . He can be charged for fraud too.
By this time the second child would have obtained a birth certificate and there his father's name will appear and it will also show that they were married at the time of the birth. (there is nothing wrong in that, despite the order of the court).
I think now the ex-wife will not contest a divorce but she should be able to obtain a substantial damage . Since her child was with the man I do not think that court would disturb that either.