Finding biological father no information

My husband and I are trying to find his bio father. He is 34 and mom claims someone as his father. We have done dna test which was negative for the claimed father. He just did a new dna with ancestry.com. the closest match we could get was 4-6 cousin. It seems the matches that don't match up with his mother are based in Mexico. We have tried to tell her that the first test was negative and he wants too know who is his bio dad. We are huge into ancestry and with arising medical issues we would like to know additional info since we have 4 children. She refuses that anyone else could be his father and won't come off anymore info. Does anyone know how to go about finding out? Really frustrated thinking she may know or should know and just isn't saying. He's even more aggrivated that we researched this mans family tree for yrs thinking he was his father and our kids call him grandpa. Even after finding out he really isn't he still lets our children call him grandpa! We honestly don't even know where to start.