I honestly think Sabo got saved by Dragon and he's been slowly healing from the injuries. I think he would've found out about Ace's execution but because they're in the new world not at Marineford it would be too far of a distance for him to make it in time and he probably had faith that Luffy would go in to save him. It's either that or he was trapped somewhere or because if his views on life which Dragon did take to heart, He is probably a high up revolutionary and had to stay with Dragon due to unknown reasons. But I think Sabo's apology to Ace would've been the 3 sake cups and the newspaper saying Luffy was still alive so Ace new his brothers are both doing fine. As Much as Dragon says they'll meet soon, I think it isn't only for a reunion of Father and Son, Maybe it's also to reunite brothers since Sabo would've talked about Luffy before.