I would sit down and have a transparent conversation with him. Acknowledge where you see his value, but be candid about the expectations that he is not meeting and that it's resulted in a break down of trust. The hard conversations are the worst, so I wish you nothing but the best. Always check in with HR and see if they have a suggested or required protcol. In whatever way that you address, make sure it's documented as a verbal or written warning/conversation.