Return to your world, but bring the little dragon with you

You can’t leave him.
You look down at the newborn dragon. His eyes, like small lava pits, blink up at you, trusting, waiting.
You throw the stone, the portal opens, and the little dragon holds your hand. The moment you step through, the world shifts. The dragon lets out a tiny sound, confused by the strange smells of your world. But he curls against you, trusting.

At first, it’s easy. He’s small, no bigger than a dog. You hide him, feed him meat, and laugh as he breathes smoke instead of fire.
But dragons grow, and they grow BIG

After some years, he’s too big to hide. It's as big as a horse, and he speaks like an intelligent young boy.

He can change his shape and become a human. He looks like a kid with ruby eyes and long black hair.

Your new life as a single dad is easier than expected! You think that you made the right choice!

You tell him stories of another world, then guide him to the countryside, where he can be freer, taking to the sky after casting his invisibility spell.

The boy proves to be remarkably cunning. He gathers valuable minerals and sunken treasures, and you live beside him like a king. He loves you and respects you as his true father.

You live out your days, growing old and content, surrounded by your five half-dragon grandsons. As your time comes and you close your eyes, your son leans close and whispers to you to wait for him; he will join you in a thousand years.

THE END
Happy Ending: How to Train Your Dragon Son