Hi Halle, first congratulations on your healthy baby.
You can do a DNA test without your DP's knowledge.
Normally, dna paternity tests are done using oral swabs, taking samples of the cheek cells. Not so easy to do without letting your dp know why you are sticking a swab in his mouth.
However, some companies offer dna testing on other types of samples for example hair, garments or toothbrushes. Start doing some research and make some practical steps to get a handle on this situation. I have found one UK company that test using hair etc. with a simple Google search www.easydna.co.uk
I think you reall need to move forward,as I say, with practical steps, rather than just an emotional tail spin which isn't helping the situation. Find a reputable testing agency, collect a sample from your DH. Like, send his toothbrush in and get the test done.
It is very very likely that this child is your DHs, but I agree you do need to test. However I would personally not say ANYTHING unless you get a test result that shows he isn't the father.
Best of luck