My son is 4. his bio dad did sign the birth certificate but has never paid child support. He has seen him a handful of times because he spends most his time in jail. He has a back ground of jail, criminal, and drugs. My husband is wanting to adopt him and I know his bio dad will not agree. So how can I prove him a unfit father and terminate his rightsd

You start by hiring an Adoption lawyer. Because you won't be doing this without one. THey will know what you will need and how you will have to go about getting them.

Yes, a lot will depend on where you are located. In America, there is no one law on this, and each state treats it differently.

In general, not seeing the child, will not be enough. The criminal record, if they are serious crimes, and long term prison sentences. In many locations will allow this to happen.

But, it is not easy and will require an attorney.