I know the extent to which Celeborn and Oropher (and by extension Thranduil) are related has never been fully established. For the sake of this story, all you need to know is that they are relatives of some variety (kinsman and cousin being used as generic terms) and would consider each other family.
With little known about Oropher's appearance, my personal preference is that he would have looked quite similar to Celeborn, sharing the typical features of the ruling house of the Sindar.
As far as makes sense to me, Thranduil is a full-grown adult here. The times when Celeborn thinks of him as 'boy' etc. are because he is older, and knew Thranduil when he was young.
Oof, that's a lot. Enjoy!
Celeborn paused outside the tent, drawing a steadying breath. There was, he knew, every possibility that the King of the Greenwood would not wish to see him. After all, he and Oropher had not parted on particularly good terms, and the acorn did not fall so far from the tree. Still, they were kin. Whatever the conflicts that had come between them, it was his duty to share in the other's grief. Thus resolved, Celeborn cautiously drew back one of the tent flaps, only to find the interior vacant.
He entered the tent anyway, perhaps it constituted a trespass, but he knew that if he put off his task any longer it would be nigh impossible to bring himself to return. No, he would wait. His kinsman could haul him out by the ear when he arrived, if that was what he wanted. Celeborn settled himself to the ground. The tent, for all it housed royalty, was spare, furnished only with a heap of bags and packs in one corner and a sleeping pallet in the other, although, considering his own habits in the past few weeks, Celeborn did not imagine that had gotten much use of late.
Ai, but he was tired. Years of feverish preparation, all in the service of days of battle and chaos that had bled into one another, lit by a fiery glow that did not rise nor set. And it was so far from over—it could be years before the siege was broken. And they had already lost so much. He was not sure he had had a proper rest since the fall of Doriath. Celeborn settled his head against his knees, his hair swept forward, skimming the ground. Enclosed in a curtain of silver, he drifted into a light sleep.
He was woken by a sharp gasp. Fighting the stiffness that had settled into every inch of his spine, Celeborn raised his head, and met the eyes of Thranduil, new named king of the Greenwood elves. The poor boy looked like he'd come face to face with one of the houseless ones, his eyes blown wide and his face gone utterly white. Celeborn suddenly realized that to wait here, in the darkness, when he was not expected, had not been the wisest of his plans. And yet, Thranduil's fingers had not reached for his knife, not even in so much as a twitch of reflex. He must know he faced no enemy, waiting to pounce in the dark. What then?
'Lord Celeborn?' Thranduil called softly, a tentative hail that was as much question as greeting, as though he could not quite distinguish his visitor's features. Indeed, there was a bandage affixed across a portion Thranduil's face, no doubt impeding his sight somewhat. And, in any case, Celeborn had been at rest with his head tucked down, the younger elf would not have been able to make out much more than the shimmer of his hair.
Oh. Oh Lady Nienna, one of the houseless ones indeed.
"Yes, it's only me." He stepped closer, looking his kinsman up and down. Besides the bandage across Thranduil's face he could see no obvious injuries, but an uneven bulk around his hip suggested more bandaging, hidden beneath his tunic, and he did not seem to be bearing his weight entirely comfortably as he stood. Some portion of his hair must have been singed off, or otherwise come to harm, for it was shorn unevenly. Under better circumstances, Celeborn suspected, Thranduil would never have tolerated such. "I'm so sorry."
The younger elf seemed to scoff, although it came out nearer to a sigh, "It was not your folly that brought us here." Celeborn held his tongue. He reached out a hand, "Thranduil, I grieve with thee."
"Oh? Do you now? And what of your lady? What of her king? Do they grieve for the people of the Greenwood? Spent like arrows in their battle?" Venom flitted across Thranduil's face, and then, suddenly, he was weeping, and Celeborn's arms were round him, and they sank, together, to the earthen floor.
He should not have been surprised, Celeborn thought wryly to himself, his young kinsman had always been somewhat mercurial by nature. He was not so different now from when he had been a tiny thing, running wild in Thingol's halls—laughing in play, then taking a tumble, wailing a moment in his mother's skirts or father's cloak, then off again. Alas, thought Celeborn, some hurts could not so easily be soothed. And so, he held his kinsman in his arms, marking time by his shuddering breaths.
Near dawn, a silence filled the tent. At last, the boy slept. Celeborn began to work his way free, wincing at the stiffness that lingered in his bones. He moved to regain a hand, one he had employed in stroking his kinsman's rough-shorn hair, when, quick as an adder, fingers locked around his wrist. "This changes nothing," Thranduil hissed, "just because you…nothing between our realms shall be other than it was when Oropher was king." Celeborn smiled, he had wondered if he would miss his cousin's stubbornness, only to find it fully intact in the next generation. "I wouldn't dream of it," he whispered, settled the young king on his cot, and walked out, into the breaking daylight.