As the next step, he would fly through the air by using the word “fly”. Of course they asked why he didn’t use that from the beginning, but he couldn’t use the same word in succession and there was a high chance he would be spotted by someone as his momentum would be slow then.
Of course there was also a chance of him getting spotted as Arnold threw him, but one would surely think his eyes are playing him a prank as Hiiro took off with quite the speed.