Agree with chippingin, it would be sensible to assume that she is pregnant with your DH's baby for the moment, until you know for certain otherwise.
Scans usually do have dates, and in some hospitals you also get a print-out with all kinds of details, e.g. last menstrual period, no of weeks of gestation, estimated due date, the baby's measurements and how these compare with the averages etc.
OW should easily, therefore, be able to provide evidence that the baby is not DH's using this kind of stuff. Or you could offer to pay for a private scan (normally under £100). Obviously you cannot force her to share information about the pregnancy, but if she refuses to provide evidence to the contrary, it does seem more likely that the dates make it possible that it is your DH's baby. If she wants to make it work with the man she is now back with, surely he will want this information too, and she should be willing to provide it to lay things to rest?
You need to get legal advice on the paternity test.
This sort of stuff is probably best done face-to-face between DH and OW (horrible, I know), unless he can simply speak to her by phone and get her to send the information by post. Maybe a friend of your choice could go with him if he does go?