does nappy cause infertility in boys   

No! Not at all. It is true that wearing a nappy raises the temperature of a baby boy’s testicles. The theory says that an increase in temperature could reduce fertility in later life, but there is no evidence to prove this theory with respect to diapers. A boy does not start producing fertile sperm until he reaches adolescence. By that time, any heating effects from wearing nappies will almost certainly be long gone. So there's no need to worry about nappies and fertility.