child born in USA by biological Canadian, then adopted by Americans, now wondering if child has dual citizenship as Canadian and American?

Adoption severed all ties to the biological parents, and no claim now to Canadian citizenship since the child has no legal Canadian parent, only the American adoptive parents.

America no longer allows Dual Citizenships.

The answer is child would definitely have had both citizenships if he had been born in Canada as they have Jus Soli.  But because he was born in the US, it is only maybe.  It depends whether Canada is one of the countries that 'looks through' adoption for citizenship purposes, more likely if child was born before 2009 as they tightened the rules then.

I, for example, was born in England and adopted by English parents but my birth family is Irish and I was able to also get Irish citizenship through them.  It was quite a lot of work; I had to get a copy of my original birth certificate, my adoption papers from the court, track down my mother's birth certificate (my father was not named) and finally get myself onto the Irish Foreign Births registry.  In your case you would have to apply for a Certificate of Canadian citizenship, same idea.  Give it a try!

If the Canadian birth parents registered the child's birth abroad it would likely have Canadian citizenship. But beyond that it sort of depends on the kind of adoption this was. Private adoptions sometimes leave the original citizenship intact.

Adoption severs the relationship to the birth parent, so no.