You really need to ask a doctor.
A woman is only fertile at a certain time of month.
Some women have acidic vaginas which will kill sperm.
Douching with backing soda and water solution before sex may help. Also douching with a mild solution of hydrogen peroxide may help. Again, you really need to ask a doctor.
It may be the man has insufficient sperm count or weak sperm. The man needs to see a doctor to check this out and see if that's the case.
You really need to see a doctor about this.