he probably wiped out the pinion clutch in the starter

disconnect battery starting with the negative cable (when youre done connect the negative last)

when you take the starter out inspect the ring gear for damage
if it sounds like its not engaging anything (no grinding) the ring gear would have major visible damage

when you have the starter out in your hand try to spin the little gear
if it spins easily in both directions the clutch is wiped out
if it is good there should be stiff resistance when you try to spin it one of the directions

the throw out fork could be bad too

either way take it to the local starter/alternator shop and have them bench test it (yellow pages - automotive electric)
they can also tell by looking at the pinion what other issues you may have
also you might want to take digital pics of the ring gear to bring down to show them