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You’re looking for Ryu no Otto Boukoku no Kami (Yaoi) I’m pretty sure.
OMG YESSS! I passed that when searching because I thought the dragon thing holding him was a slime monster or tentacle
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It's a yaoi manga, specifically black and white, where the MC is a soldier for his kingdom, and his brother is the king (or emperor). The MC ages normally, but his brother ages extremely slow, so the MC looks way older even though he's younger. The ML is a dragon (or something), and he takes on the body of a soldier the MC killed in war. He likes the MC, so he shares his lifespan with him, and eventually the MC outlives his brother, when he thought he would die way before.
Extra details: His brother meets him before he dies, and the MC is surprised that his brother who'd didn't age when he (the Mc) grew up, is now an old man on the verge of death.
If anyone can find it... thank you xx