According to this New Zealand promo for the big Arrow and Flash crossover event Oliver will discover that he has a son.
It was also announced earlier this year that Connor Hawke will be appearing on Legends of Tomorrow. In the comics Connor is Oliver's son.
In the promo Oliver mentions that his son's name is William meaning it wont be him we see in Legends of Tomorrow but, could his son have a bigger role within the story? Could he be the one seen in the grave during the flash-forward in the season 4 premiere?