If you can kiss him for 30 seconds, then try kissing him for a series of 30 second kisses. You can take little breaks between the kisses, and you may find that that helps alleviate your tension. I’m not sure from your post what you feel that makes you break away after 30 seconds, and that would help to know.
So, see if you can work up to six 30 second kisses, and see how that goes for you.