First, Wausau is the County Seat of Marathon County Wisconsin.Try the Register of Deeds at the Marathon Co. Court House.(Sorry I don't have an address or telephone No., it's been 50 years since I lived in Marathon Co.)I hope you have a fairly narrow time frame on his death.It will help with their search.You should be able to get a photocopy if his death record.
Second, try Pine Grove Cemetery in Wausau.It's the city's largest general cemetery.There are a number of church cemeteries in the area, but Pine Grove is the major general cemetery, and a place to look.
Third, Wisconsin Vital Records services. http://www.dhfs.state.wi.us/vitalrecords/index.htmhttp://www.dhfs.state.wi.us/vitalrecords/index.htm This might give you a broader search for your ancestor's death.You should also be able to get a photocopy of his death record from there.