Funny thing is that I am dealing with the exact same issue with my son. Just allow him to finish out the season despite the fact that he may dread attending practices and going to the games. You want to teach your son the importance of completing tasks even when faced with adversity. You are exactly correct in the fact that you do not want to teach your son to be a quitter. He must finish what he has started and then may want to change to another sport. It will be tough watching him play football when his heart is not really into it. But in the long run, it will be a definite learning experience for you all.
I hope that I have helped. Please ACCEPT. If you need further assistance, please ask!!! Have a great evening!!!