Yes, the sooner the better, call him. It's only proper to be honest with others. If your relationships are based primarily on sex, they will not last long but you have the right to pick and choose whomever you want. Don't string along the other guy or procrastinate letting him know how you feel. You don't need to make up any excuses or give a specific reason if he asks you why. Just say you feel it better not to date him at this time. The fewer the words the better, he'll get the point. Remember, what goes around, comes around. Happy New Year and Good luck!