Chiameron can only be tamed when he's under 20% health. If you're high level you'll have to be careful you don't kill him. If necessary remove all your equipment.
*Level scaling: In Shadowlands most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.