My son was 6 yrs old and he was told that he has -0.75 cylindrical power in both the eyes. Will it increase? How to reduce it?

Increase likely. Your child has a mild astigmatism. This is common and is fairly stable but might change a little over time. Your child's astigmatism is low enough that for a while nothing need be done in terms of eyeglasses. Just have him checked in a year and see if it has changed.