Greetings... my fiancé wants to adopt my 14 year old son. He's been in his life since he was 6. I keep reading that I need his biological father's consent but what if his biological father's dead?

Then his consent is pretty easy to get.

Sorry... I don't mean to make light. If the biological father is deceased, it's a very easy process. You will need to be married in most states though.

I believe all you would need is a death certificate. Do you get Social Security death benefits for your child? If so it should be very simple to prove that he is dead. Death certificates are public record.
For an expert to answer your question completely they would need to know things like do you have proof of paternity if you weren't married to him, maybe even what state (s) you are dealing with because I think adoption laws very by state.
I am not a lawyer, just some life experience with this type of thing.
Hope this was helpful to you and best of luck to you and your new family! ;o)

If the bio father listed on the birth certificate or though a DNA test or some how reconised thought the courts, if so it is much easier.

There could be some challenge by grandparents but I doubt it would stop it.