I agree that being honest is the best way and there is really know way to know how to let a guy down easy. For one, how one might react may be entirely different and there may be nothing you can do to keep him from reacting as if he'd been crushed by an elephant. Unfortunately I do not have good advice here because there is no single answer for this without knowing the other person, the situation etc.

but go with honestly always. And in general, I'd always just try to be as empathetic and compassionate as you can about revealing this information, unless the guy is a lech and you have no problem pushing him away in a more aggressive manner XD