Answer: Before talking about what you can do for your son, I would like to address what you need to do in your relationship with your son.
Your love for your son should be unconditional and should certainly not be predicated on his ability as a football player, as an athlete or as a student. By calling him a sissy you are telling him in effect that his worth as a person is related to his performance or lack of performance on the football field. Unfortunately that is a very difficult "bell to unring". What you said has probably had a devastating affect on your son's feeling of self worth. It is even more devastating because it came from you, his father, his mentor and someone he has been taught to listen to, look up to and respect. This is something that will be quite difficult to counteract. You will need to work very hard to make him understand that you made a mistake in calling him a sissy and that you did not intend to hurt him. That you recognize your mistake and "feel bad" gives me hope that you can change the nature of the flawed relationship between you and your son when it comes to his participation in organized sports.
The "potential" that you say he has may just be a manifestation of your expectations and, based on your email, your expectations need to be altered. First you must realize your son is a child not a small adult. He should be participating in sports for his own fun and enjoyment not for your gratification.
Just because he is bigger and faster than some of his peers does not guarantee his success on the athletic field. Unfortunately misguided or improper expectations are common faults of parents and coaches in the high pressure world of youth sports today.
Parents are often guilty of what experts call "achievement by proxy" living vicariously through the successes of their children. You must figure out how to allow your son to participate in sports on his terms, not on yours. Ask yourself "Is my child playing football because he really wants to or because I want him to?" Obviously, he should be playing because he wants to. While it is perhaps encouraging that he wanted to go to practice after this incident, it is also likely that he is doing this because he believes you want him to play, not because he wants to play.
I would start making amends to your son by explaining to him that you love him unconditionally - whether he likes football or not and whether he is a superstar or not. Tell him that you want him to have fun and to enjoy himself while he is participating in sports. You should try to get him focused on the fun aspects of playing without relating it to his tackling, blocking or running ability. Try to encourage him by praising him for his effort and his determination. Look for things he does well and keep reminding him of those things. Talk to his coach and explain that your son seems timid in tackling, blocking and running. The caring coach will try to match your son up with players who may have similar characteristics or feelings. Once players find out that tackling, when done properly doesn't hurt, they usually become more aggressive. But this takes time and encouragement by the parents and the coaches involved. It doesnąt happen over night.
You did not say what type of program your son is involved in but I would hope it is a recreational type program that recognizes and accommodates the wide physical, emotional and maturity differences in kids of the same chronological age. This recognition and accommodation is not generally found in highly competitive or elite travel sports programs. He may be in the wrong program.
You need to keep your desires/expectations in check in regards to where and how he participates in organized sports. From the sound of your question that may be difficult for you.
I would recommend very strongly that you read a book by former NBA basketball star Bob Bigelow titled "Just Let the Kids Play". It could help you understand the negative impact that organized sports, with demanding parents and coaches can have on children.