If a boy's chest is big like a girl... is it anyway to reduce it..?

Yes: ~1/2 of males experience transient enlargement of breast tissue under one or both breasts during puberty. In most it will subside within a year but sometimes takes longer. Kids who are overweight are more likely to lay down fat in these areas which tends to persist. Passage of time & weight loss are the simplest solutions. A few will need surgical removal of the spongy & fatty tissue.