My bf suffering testiscular cyst, its like a big mango size.is there any cure or somehow reduce the size?

Is there a diagnosis: Has he been formally tested and diagnosed with a cyst or is this what you and he feel that it is? He may need an ultrasound to know for sure what is causing the swelling but it could be a few things, such as fluid collections between the different tissues of the testicle and scrotum. Many of these things go away on their own, but if big enough may need medical attention. See a doctor for testing.