I see now that this forum is useless. Thanks for all of the help. I will strive to be as perfect as you all are. In the meantime, I will work on telling my boy the truth. Whatever mistakes I've made, I'm just trying to do the right thing.
Look, the guy is dead. And personally, I don't know anyone with a functioning time machine. So, moving forward, I suggest you meet with a lawyer and discuss what can be done in your situation.
Even if the biological father were on the birth certificate, at this point, with him being dead, your husband could do a step-parent adoption.
I don't know if it would make any legal sense to disestablish your husband's paternity so that he could legally adopt your son. I don't know that a court would even think it in the child's best interest at this point to disestablish paternity, given his age and the lack of a living father to establish paternity with.
Absent a court order, you are under no obligation to consent to a DNA test.
So, meet with an atty and get this clarified, and take care of what needs taking care of.
After you get everything straightened out, toss together a will detailing what your wishes are for guardianship, should something happen to you.