It's not a good idea to be kissing a herp in the first place......but think about how YOU would feel if you saw a mouth that is much, much bigger than YOU coming toward you....you would think you were about to be dinner!!!
Just follow Michael's excellent advice....hold him in your arms and just "cuddle" and and gently, quietly stroke him. He's scared, you need to gain his trust. Eventually if you keep that up, he will learn to trust you.