How to turn him on?!?

I've been with my boyfriend for almost 4 years and I've been try to get him in the mood and he just doesn't care. For example: he says he likes my butt so today I put on really tight short shorts and walked past him, he didn't even look at me? And then I tried kissing him passionately to try and start his engine but he laughed. I don't understand why he doesn't want to... All I know is I'm getting very frustrated and It makes me feel unattractive or not sexy enough.... Any tips??