Look for ideas to reinforce the trust. Maybe talk. Maybe symbolism.
When you tell him you trust him, ALWAYS maintain eye contact. If he blinks or looks away, try to regain his eye contact so he sees your words come across your lips.
Symbolim. What act or object could represent your trust for him. What could you hand him to carry as a reminder of your trust, that he may need to give back to you when he returns. This repeated process of acknowledging trust each time he leaves gets planted deep in subconsious. It may be an object or charm that goes on his keychain or in his pocket. You just don't want it to be easy to lose, accidentally. That would make you both feel bad. Is he into neckchains, bracelets, or pinky rings? Something as simple as a note for his wallet even. See if he doesn't recipricate with his own symbolism after the first time.
