Teens, and advice?

I am a dad to a 14 year old son. He is pretty responsable, but when our naighborhood had just played a game called pickel, where i person hides, and if you find them you hide with them, and the last person to find them looses, me and a girl who is 16 were hiding, my son came, hid with us, and when the girl said she had to pee, he made waterfall sounds, and then the girl, actually wet herself, accadentally. Well, since then i think he has had a crush on her. Then recently while playing manhunt, him and her decided to play a joke, and layed behind a hill and took off their cloths, everything, and swapped cloths while laying in semi darkness next to eachother. They swapped everything, boxers, bra, EVERYTHING. It was sapposed to be a "joke" to confuse everyone. Should i be concerned, or let them be. My son seems responsable, but i have to admit, even as an adult, the girl (two years older than him) is very hot. Should i be concerned?