Aww man, i know how it feels to be guilty about it... but seriously, you cant be totally super-safe all the time... after all, where would be the fun in life?
For instance, when in Slovakia my cousin managed to hit me in the hand with a woodsmans axe... we where just mucking around and he missed, catching me a blow with the blade at an angle.... sure i felt kinda resentfull to him at first, but now it's just a story i tell and we both get on just as well as we did before
Anyway, so long as he isnt badly injured, like you said it could have been worse... jugular or eye would have lead to somthing permanent... and as for him getting you back? He's your friend, he should realise it was a mistake, an accident and really i dont think he'l be trying to 'get you back'....
So anyway, just make sure your extra-nice to him in the short run, and really i think it should be okay.... I'm just glad to hear he's okay and that it wasnt in the eye, that would have been the worst outcome