She will have made a CMS application, but as part of that she will have answered a question about how many other children he is responsible for. If she doesn't know what the answer to that question is, the claim that they're calculating may be based on incorrect information.
Go to the website and see what that calculates the ongoing payment to be based on your actual circumstances.
That is one thing that is potentially able to reduce the claim at least.
In terms of a backdated claim, a google seems to suggest that CMS cannot backdate payments from before a case was opened, so if they are claiming for years' worth, then it would suggest that they have been aware for a number of years and your DP should therefore have been aware also...
Not suggesting he's definitely lying to you OP, but if it's true that CMS can't backdate claims, then this claim has already been going on for years, without your knowledge at least. (Any prior year child support would appear to be a civil matter)
I would firstly suggest a call to the CMS with both you and your DP in the room, on speakerphone, to ask where this claim has come from and why it's been backdated particularly, then saying that you are going to question the parentage and request a DNA test.
As PP have said, until you know for definite, there's not a great deal more you can do at this stage other than be prepared.