My grandfather was adopted and I want to know who my real great grandparents are. I need help!

My grandpa, Henry Redeker, was adopted when he was young. No one knows who the real birth parents are. The only information I have is that his last name would have been Weber. He was born in Wisconsin in 1925. His adoptive parents are Minnie and John Redeker. After doing some research, I potentially have a match to who his birth parents are. I checked out the 1930 census and discovered there was a Henry Weber Sr. and Regna Weber with a son named Henry living together. Henry would have been 5 years old. Henry Weber Sr.'s dad, Mathias Weber, age 75 also lived in the home. They were farmers and rented. I figured they lived a hard life and possibly gave their child up for adoption because when I checked the 1940 Henry Weber no longer was listed with Henry Weber Sr. and Regna. However, there was another son listed who wasn't on the 1930 census, named Eugene. He was born the same year as Henry Weber, 1925. 

On the 1930 census I also checked for a Henry Redeker. Henry Redeker, about 5 years old, lived with a John and Minnie Redeker and a hired man. 

So here are my questions:

1.) Is it possible to be listed twice on a census with a different last name? Could it be that Henry Weber was adopted in 1930 as the census was being taken and therefore listed twice?

2.) How could Eugene be listed on the 1940 census, but not the 1930 census? He would have been 5 years old during the 1930 census.

3.) How can I be certain that I'm making the right connections. Is there a place where I can find adoption records during that time? 

Any information anyone has I would greatly appreciate. I would love to be able to share with information with my dad.

Thank you!!!! :)