My husband has a son from previous marriage, how can I adopt him?

Were is the mother of the child because if she's around she has to consent to this adoption and her rights will have to be terminated.

Yes, you hire an attorney, get the bio mother to sign over rights to allow adoption,

Your husband will need to contact his biological mother, she will need to sign over her parental rights BEFORE you can legally adopt him.
If she does this, then you can file the papers and go through with the adoption. If she does not want to relinquish her rights, then unfortunately you will not be able to adopt him at all.