Oh my and I thought I had my hands full.
First off, You're putting up with an awful lot just to have a decent tech. But remember, YOU are the manager and the person in charge - that needs to be made painfully clear to him.
Call him on his stuff immediately. Tell him so-and-so just told me you are claiming I am stealing. Why are you saying this? And then wait for his answer. And then you are going to rip him apart. You're going to do this because that's the only option you have. He's going to backpedal, make up more lies and try to deflect blame. You need to put your foot, or 2 inch heel, down and right on his throat and make it clear he is never going to say ANYTHING about you again. Let him go running to someone else after that and see how much sympathy he gets. HR coming to you to tell you that YOU are out of line? All I can tell you is if that were me, he'd wish he'd never opened his mouth.
He's either going to get in line or he will quit. I know you need a good tech but this guy is not worth it. This person has problems, big ones.
And why, you ask, am I giving the advice like this? Been there. Doing the nice gal thing, treating him kindly, bla bla bla. That's just telling him he can walk all over you and everyone else.
Whatever happens, would love to hear the outcome. As you may recall, maintenance is of particular interest to me right now...