Not sure you will have time before he turns 18. If he turns 18 he will not become a citizen. His parents have to give up their parental rights and you have to pass the home study. Then you need a lawyer to draw up the papers that every signs and then the lawyer get it approved in court.
by adoption
no
No. A 22 year old is an adult and, as such, is too old to be adopted.
No. In the US adults regardless of their citizen status cannot be adopted. A legal immigrant can be sponsored by a US citizen for permanent residency and citizenship.
It depends on the state where the US citizen resides. In most states, you can adopt your 20 year old stepson in an adult adoption regardless of his immigration status. But you should know that you will not be able to use the adoption for immigration purposes.
Yes, adults can be adopted. You will need to consult a lawyer but it is fairly common. HOWEVER, the 19-year-old is an adult so they cannot be adopted "single sided," that is, without their consent. (Note that a non-US-citizen 19-year-old cannot come into the US and become a citizen through adoption the way a younger child could.)
If you plan to do this to make him a US citizen though it will not work. Someone who is 18+ will not automatically become US citizen when adopted. Only children do.
No, to adopt a child you have to be at least 18 years old.
No, most places require you to be at least 21 to be considered to adopt someone.
You can't adopt an adult. And his child is not an orphan, so you can't adopt him either. You can marry the man and then adopt his son.
Yes, you can adopt at any age above 18
No, a minor cannot adopt another minor. And most jurisdictions don't allow adoption under the age of 21.