thats a hard one to call.
for me, id say you made the wrong call - youve give the FK for the way he went in, which would be for serious foul play surely?
again, if it were me, you have 2 options:
1. play on, he one the ball perfectly, didnt catch the player at all - but maybe have a quiet word in his ear that next time he might not be so accurate
2. give the FK for serious foul play, as you have done, but then id have stuck to my guns and the LOTG and sent him off too
the arguement you have for the red would be that the Law does state that it is 'kicking or attempting, jumping or attemtping, punching or attemtping....' so you would class it as a clear attempt to make a forceful challenge, had he connected then it would have been out of control, horrible, and potentially a nice shattered ankle !!