Need help encouraging boyfriend to stick to his morals.?

My boyfriend (Christian and a virgin) has this "rule" that he promised God this that and the other, however the general gist is that he will not have until he is 21 and even then until he has found the right person. I respect this and feel really nice when he tells me he things that I'm the right person. However, every now and then he'll say stuff like "Gah, I hate myself right now. That stupid rule" and will get really frustrated. I really don't want him to do something he'll regret. Just wanted some advice on how I can encourage him to stick to his "rule". I really really need advice and all answers will be appreciated.