For the mom who has the brother in law in prison:

I was around six or seven the first time I watched my older brother get taken out of the house in handcuffs. It was chaos as my brother was screaming out of anger and I was screaming out of fear of being taken away and my parents tried their best to figure out how to be a good parent to each of us, eventually I became used to it. I can’t count how many times I did homework sitting at the visitation tables waiting for my brother to come over to talk with my dad and I.
I think it’s incredibly important to be honest with your son in an age appropriate way (absolutely use the word prison, don’t necessarily go into detail about what he did to get there) and I think that it’s really important not to refer to the prison as a school, because if something happens when you and your son are visiting, you don’t want school to become a place that he fears.
There are multiple books for children who have incarcerated parents that I know could really help you with talking to your son about his uncle. Here’s a link to one list: http://www.nh.gov/nhdoc/fcc/books.html
There is also a special Sesame Street episode in their Little Children, Big Challenges section about incarceration that you might want to check out.