If a man paid child support for 15 yrs and begins to question paternity can he get this stopped

I'm not a lawyer by any means but if a man pays support, especially unwillingly, for a child that's finally proven to not be his, he should get all of his money back, promptly. Just my opinion, by the way.

It will depend on your location. In the US, there are some states that will not allow you to even request a DNA test after that many years. In other US states, yes you can request a test and if proved not to the be father stop the support payments.

But after 15 years, and I would hope 15 years of visiting "your" child" you should have some emotional bonds with the child.