If you are both adventurous types, when you get down to his scrotum, keep going! Just make sure he has washed first! I have dated guys who swore that blowjobs were the best thing ever - until they had a girl rim them! I had one guy compare it to a religious experience! lol
Make sure he is already very relaxed around you though - it's a pretty intimate thing to do (even if you have had lots of sex previously and seen them naked). Get him to lie on his back and start to give him a blow job, then work your way down as if you are licking an icecream cone! Every guy I have dated has been extremely sensitive there and when they relaxed enough to let me do that they reeeeeally enjoyed it (once they got past the stupid 'ohmygoddoesthismakemegay' thing)! No, being rimmed by a woman does not make you gay - being rimmed by a guy, however...! You see the difference? lol
But seriously, give it a go. Just make sure he is clean first - you have to be in a whole other league of kinky for that!