Did Dr. Ben carson say he has performed in vito surgery and fetus experiences pain at 28 weeks

Operating on babies still inside the mom is an option that is being studied extensively. There are many birth defects that this procedure can fix early in fetal developement that will give the child lifelong benefits. Some of these birth defects can even cause the baby to die at birth, but if the problem is fixed before he is born he will live.
Doctors are actively researching the question of anesthesia for the fetus during these operation. Most of the experts do believe that a 28 week fetus does feel pain. Most of these procedures are done with Mom under heavy anesthesia, and that is taken by the blood to the fetus also, so the baby is as deeply asleep as the Mom is.
There is interest in finding less invasive (less traumatic) ways of doing these procedures. Just as these simpler procedures are easier on the mom, so are they on the baby. Doctors are finding ways to use simpler anesthesia on the Moms, then using safe medications to give the baby complete pain control also.
It is an exciting development in fixing birth defects and giving many of these people lifelong benefits after birth. The idea of fetal pain is taken into serious consideration by the doctors, nurses, psychologists, neurologists as well as people who consider the ethics and the long term effects on these babies. As an obstetrician myself, what is happening with the baby is ALWAYS a consideration. From playing classical music to the pregnant mom's tummy, to the possibility of being able to correct problems that would otherwise effect that individual for their whole lives.
If you are interested in reading more about this here is a place to get started:

Fetal Surgery: Huge Strides in Care for the Tiniest Patients ...
https://www.yalemedicine.org > stories > fetal-surgery