Try to speak to him, over the phone or face to face, if possible. And just say you're sorry, you didn't mean to hurt him.
There's not a lot more you can do than that if he doesn't come to accept your apology and move on.
After you've made your apology, move on yourself and chalk it up to experience. It may be that in time, he gets back in touch, maybe he never does.
Learn from this, as we all learn from these sort of things and DON'T be too hard on yourself. Everyone at some point in time has said something which has upset someone else.