It would be completely wrong for the female to perform the test without his knowledge that it is about to take place. In a caring, trusting relationship it is only appropriate that such life altering adventures be taken together by talking, buying the test, and he there by your side to view the results. If you feel the need to take the test behind his back, one or both of you are in the wrong relationship and should have kept your pants on to begin with.
That said, your scenario more than likely would result in you proving it is his, anyway. In most cases, a DNA test is required before child support is granted if the man has even the slightest doubt.
If you can't trust them to do the test alone, then it was never a caring, trusting relationship in the first place.