No not William, rather 怪獣王子 (Kaiju-Oji) and his famous yell, オーラー (o-raa)! I've never seen this before and just randomly came across the YouTube now but it looks to be right up my alley. I can find little info in English on this, but it looks to be a 1967 series about a kid who fights dinosaurs with his pet Dino & a trusty boomerang. (Sound a bit like Dino Boy anyone?)
This very odd image is done by a fan of the show and does really creepy stuff with big-headed dolls! At almost 800$, I don't think I'll be picking up this box set any time soon!