From my understanding, Wisconsin enacted a law formally called the "Wisconsin Grandparent Liability Law". This law and its implications are often called "Grandparent Liability". Basically, this law states that after the identity of a minor's child (in this case, your son) is determined by the court, the minor's child (your grandchild) is entitled to equal financial resources from both parents. Since your son is considered a minor and, in the eyes of this law, you are obligated to care for his needs as a minor. Since, under Wisconsin law, one of the needs of a father is to provide support for his children, if he cannot provide such assistance, then it is your responsibility to assist him in fullfilling his needs (obligations). From my understanding, your son's girlfriend is 18, so her parents are no longer obligated to care for her (and hence, her child). However, if the child was born BEFORE she turned 18, then your son's girlfriend's parents would also be equally responsible for support up until the time your son's girlfriend turned 18.
This situation does not seem as bad as it could be. You are required to help pay for expenses (i.e. medical bills) only up to the point when your son turns 18 (after that, he is no longer your dependent). If your son's girlfriend was under 18 at the time of birth, then they to would be retroactively responsible for costs of all events (care, medical expenses) equally until the time she turned of age - at this point, she is responsible for her portion of the child care.
I know this sound's a little confusing, and I would strongly suggest you consult with an attorney before you sign ANYTHING. If there is even the slightest chance that your son is not the father, then, of course, there is little chance of any financial obligations on his (or your) end.
As far as your son's girlfriend being on birth control (or not) - this fact is irrelevant. Not only can birth controal fail, it is assumed that when two people of the age of consent engage in sexual intercourse, they are aware that there is some chance of pregnancy.
Lastly, for your information, this law was passed so that parents of minors would have a stake in preventing their children from having children basically you are repsonsible for your child's sexual actions. Lawmakers hoped that if the parents had an incentive to prevent teenage pregnancy, the teenage birth rate would drop.
If you want to read the complete Grandparent Liability law, you can go to this web address:
The actual law is contained at 49.90 (they are in numberical order).
I hope this helps.