Can I adopt my stepson if he’s an adult?

My step-son and I have been close since he was in middle school and we talked about me adopting him several times. But, even though he never had much of a relationship with his biological mom, he didn’t want to upset her by terminating their relationship. She passed away last year, and he’s approached me again about an adoption. Even though he’s in college and an adult now, he says he’s always considered me to be his real mom. I’ve also heard that if he’s my legal son, I can leave things to him in my will and avoid certain estate taxes. Can someone adopt another adult and does it matter that his biological parents are not around to consent?