My condolences on the death of your boyfriend. I assume he himself never had the chance to do a DNA test for the benefit of confirming the child is his daughter?
How did you go about testing your boyfriend's parents? Did they go to a reputable lab along with you and the baby, or was this done with a home test? Were you there when they did the swabs? Did you observe the handling of the swabs until they were out of the control of everyone personally interested? (such as, until they were in the lab technician's control, or in the mail)? And, do you know for sure your boyfriend was not adopted? If your boyfriend has a sibling and you know for sure they are blood siblings, you could test with the sibling also.
The problem with the idea that the guy from early February is the dad is that the dates are way off for that to be true. Besides, he had a test through the DDC that says he is not the dad. [Did you go to the lab with him and witness him doing the swab and handing it over to the technician?]
Your 5w4d ultrasound suggests the baby was conceived around March 31, as does the estimated due date they gave you. (Having a baby in mid-December doesn't change that, babies do come a week early sometimes.) In your fifth week, they would not have looked at a baby that was conceived in the last week of March and confuse it for a baby conceived in the first week of February. If the first guy had been the dad the embryo would have been many weeks more developed.
Unless you had sex with someone else between February 8 and late March, the baby is from your boyfriend. In your shoes I'd ask your boyfriend's parents for another test, and/or a sibling of your boyfriend for a test, and do it in an official manner at a well-thought-of lab, with witnesses to them doing the swabs. If you think they are trying to get out of this child being their son's baby, have your lawyer handle all discussion about the DNA testing. If the situation is secretly hostile on their part, none of this should be you just talking to them and having a drugstore test done. Or, if you think the situation is friendly but that a secret is being kept regarding who your boyfriend's biological parents are, leave space for one of the parents to draw you aside and whisper in your ear about why the test came out negative. But I would guess that if the parents loved their son and are grieving his death, they would be pleased he had a child, and wouldn't be trying to avoid a correct answer.