How to give my boyfriend an erection when making out?

He challenged me to give him one lol don't ask :x but how can I make him hard while making out without taking any clothes off or touching his crotch?

Answer: 
Honestly, its not difficult. When making out, push your body into his, lightly brush your hand against his skin (doesn't have to be his crotch). It shouldn't be too difficult. My boyfriend gets an erection just from making out, never mind anything beyond that.