Recently I adopted an adult, prior to the legal adoption he fathered a child does the legal adoption still obligate him or does it terminate as with a child who has been adopted?

Your relationship with him does not affect his relationship with his child.

He is still obligated to the child.

I asked this question because it appears that once a minor is adopted the natural ties to his biological ties sever thanks for the reply.

If a minor is adopted, his biological ties to his parents (and subsequently the families of his parents) are severed. Even if a minor has children and is adopted, the minor remains responsible for his or her offspring.

It works going up (severing parental rights and responsibilities of the adoptee's parents) but not down (adoption of a person will not affect their parental relationship with any children they might have).

Hope that makes it more clear!

Congrats, you are now a grandparent,

Hope this adoption was not a ploy to get out of child support, since it will not work.
Also since adult adoptions are rare, why was it done?

But he will owe child support, it does not stop it.

Any question on law needs to include your general local as laws vary by area. But the answers have been correct. However, not every place allows adult adoption. So I'm curious as where this is and the reason for the adoption (usually its only for inheritance). And that bears on your question as now this child could be in line for an inheritance.

From the ID, I'd guess maybe OK, which does allow adult adoption.