My ex-boyfriend thinks he fathered my child and he wnts to know for himself but he's in prison now

That looks like another right he has surrendered.

How about the other candidate(s)?

You can still get an idea by elimination of the other guy(s).

You or he gets a court order for a test.

Agreed with Scott, with a court ordered test, the prison medical staff takes the sample and sends it to the lab