I practice in Michigan where the statute for "DUI Third" carries anywhere from one to five years in prison. However, sentencing guidelines actually permit one to receive a sentence below one year. In general, most jurisdictions punish this offense as a felony with a prison term. Since you highlight that your son's dad has a lengthy criminal record, he should probably expect to receive a very lengthy jail term and even a possible prison term. Either way, it will almost certainly be longer than 30 days.
There is absolutely nothing from stopping you from pursuing child support right now. However, since your son's father is about to become incarcerated, he would be entitled to request to have any child support order abated while he was incarcerated. This does not mean that you cannot seek and obtain assistance from the state for your son and yourself. The state would pursue recovery of this support from your son's dad upon his release. You can usually make a child support claim through your county prosecutors' office. Under any circumstances, though, as soon as you make a claim for assistance, there will be inquiries as to the identity of the father.
There are sentencing guidelines for all OWI offenses in Wisconsin. He is subject to the 9th Judicial District Guidelines, which call from anywhere from 30-365 days in jail depending on several factors.
His entire driving record will be a factor, his BAC will be a factor, whether children were in the car will be a factor, and also whether he was cooperative with officers. There are other factors, but these are the main ones. His sentence will depend on how he fares as to those factors.