Is Toothless male or female?

Novels

The original novels clearly indicate that he's male. Note that he's referred to as "he" by both the characters in the book and the omniscient narrator

Hiccup decided to take the opportunity to check on Toothless's basket. He pulled it out from under the bed. A thin plume of bluish gray smoke was drifting out from under the lid.

Fishlegs whistled. "He's awake all right," said Fishlegs.

and

"Oh shut up, Fishlegs," said Hiccup, impatiently, "you're frightening it."

Toothless huffed and puffed and blew out some smoke rings. He inflated his neck to make himself look bigger, which is something dragons do when they are scared or angry.

Films

Where the films are concerned, we have both in and out-of-universe quotes that show that he's a male;

From HTTYD2's writer/director; Dean DeBlois

"I'm halfway through the script. It continues to track Hiccup's coming of age, but it's also equally shared now that Hiccup and Toothless are chiefs of their clans. The story also follows Toothless' storyline, and I think audiences will like that because they seem to respond well to Toothless' antics and what seems to be going on in his head."

and from the original film script for HTTYD1

Amazed, Hiccup sits up and tries to touch him. Toothless HISSES and flaps off to a crash on the other side of the cove. He BLASTS the mossy ground to a red-hot temperature... and curls up on it like a giant dog.

The Wiki mentions that he is male.

Plus the dragon is referred to as a 'he' by Hiccup several times in the movie as well as in the books by Cressida Cowell.