Basically what it says on the tin. Instead of a Catalina, the baby comes out a boy - or, perhaps, given that Maria of Aragon was a twin, Isabella gives birth to two living twins this time. Regardless, the little infante is likely named Ferdinand or Alfonso, and is heir once Juan dies.
So how does this alter the fates of Spain and England, not to mention people like Charles, still likely to be Holy Roman Emperor and yet nephew to the King of Spain instead of inheriting there too. Who would Alfonso (I prefer that name for less confusion) marry, and our Tudor princes for that matter? Does a little less sorrow and worry extend Isabella's life? Would a teenaged King Alfonso of Castile clash with Dad after Isabella's death?