It sounds as though you've made him nervous - maybe afraid of losing you - so he's become skittish about discussing the issue. But as has been said many times here, communication is crucial. It's crucial to any relationship, but especially in a D/s relationship and even more so in a long-distance relationship. Both of you need to just suck it up and put your respective truths out there, regardless of consequence. If you both really care for each other, the relationship will survive.