problems with employee under me

Hi I'm John-

i am a sales manager and our company hired a technical support guy a year ago. We decided to move him into sales next year but as of late he is being pushy very disrespectful of me, visiting customers without telling me and trying to hijack some of my opportunities. he doesnt have an ounce of sales experience. i even set up a plan to have him trained by the end of the year but he continues to be a problem for me. i feel like he is greedy and only cares about money and is trying to do everyones job. I dont think i can get him fired at this point since hes basically a nice guy and my boss likes him but I see him causing more conflict and creating a hostile work environment moving forward. Any suggestions on how to handle this? thanks so much