Best paternity tests without parties aware?

My brothers baby mama is claiming that she needs to be induced at 34 weeks because baby is measuring “too big” at 5 pounds. She has claimed this entire pregnancy that baby is measuring further ahead and will be coming early. My brother is a complete idiot and doesn’t question nor understand a single thing about pregnancy and all. He just believes every word she said. They weren’t even sleeping together till the end of May 2018.

So how can we get a paternity test without either or their knowledge so we can prove it’s not his kid?