If Roy fails and Blood Oath will be carried over to Remade World, with Xykon dead by the hands of third party - Eugene is stuck on the cloud forever, right?

Or if he is getting kicked out of Celestia due to shifted alignment and Roy gets there - he might never see Roy again, probably, no?

All the good reasons to pay one last visit to his son.

And if he is so good with scrying and has literally nothing else to do but watch and think, then he saw what Roy is doing, i.e. that he needs to unlock his sword's power and in what way.

All the good reasons to pay one last visit to his son and ANNOY THE CRAP OUT OF HIM!

Because if Eugene just tells Roy to "just remember how you hate me, your father", it will make Roy feel less righteous anger for him, since he would be both helpful and admitting his flaws to a point.

Eugene probably did the last and best thing he could do for Roy, by doing the thing he is best at.