Most boys with KS/XXY have reduced fertility.
Fertility may diminish further as they get older because the number of sperm produced can decrease when ageing into adulthood.
In light of this, some boys with KS choose to freeze any sperm as a teenager to preserve it for family planning in the future. It may be important to have conversations with your son in his teens, when it feels right, about having a family to ensure he is aware of his fertility and his options.
Reduced fertility does not mean a person cannot have a child. Natural conception may still be possible in some cases but if not, an assisted conception method can be successful for some men with KS/XXY.
If he does want children, other paths for him to consider are adoption or sperm donation.
It may also be helpful for your son to be aware that lower levels of testosterone associated with KS/XXY may result in a lower sex drive.