It hurts when he tickles me? haha?

ok so this guy likes me and whenever i see him he tickles and pokes me in my sides or something like that and he's really strong and I'm kinda tiny and he does it too hard lol i litterally have like bruises on my sides from him i try to tell him that it hurts but it's usually when I'm laughing because I'm so ticklish so he never really believes me but it hurts at the same time I'm laughing!! if that makes any sense lol so what should i do??