it sounds to me like the dog does'nt know what to make of foxes yet, or he just simply can't catch one, theres far more dogs capable of killing foxes than there is capable of catching and killing them.
i dont think the dog is at the stage where he is scared of foxes if he is snapping at them, providing he has never caught one yet, so he should'nt have been bit or spoiled either.
its far easier to enter a dog to foxes when there working along side an experienced dog but if thats not possible, wait until september and get him on some cubs and plenty of, just to build his confidence.