It would depends whensaid kid is born since a 1679/1680 birth (most likely, as Mary's last possible pregnancy is dated to that time IIRC) probably butterflies the Glorious Revolution/Jacobit as we know it.
Although he wouldn't be the Jacobite heir. Williamite, though.
If a son is born to William and Mary in '80, let's call him William, then presuming that history proceeds as OTL until the Glorious Revolution, the 8 year old William will likely be made Prince of Wales.