In most places, it is not legal to swim without swimming attire. However, there are nudist beaches in some places in the U.S. and in the world for those who wish to swim without attire.
Well how old is he? Why does he want to wear panties? Does he like the cotton ones or the silky nylon ones? Does he get an erection when wearing them. Does he want to wear other female items or...
If your son i talking to you you should be flattered and be acceptng most sons hide it in shame you should tell him not everyone is as accepting so he should refrain until you both are comfortable...