Did you see this happen yourself? I mean did you see the coach shake his head all over with his helmet? I don't think a coach would do that even at the high school level, and especially not in front of a parent. If the coach did actually act this way, then if you can even get him to go to practice again, have your husband or an uncle stay at practice with him. No moms please! We moms are way too emotional to handle a rough sport like this, lol.
We had an LD son also, he was in Tae Kwon Do and earned his black belt, though it took him longer to go thru the belts than other kids.
Many times an LD child excels at a sport. Or even most sports, and is one place they can really shine amongst their peers. No one cares if you don't read well, or if you have a hard time in math, if you're really good at a sport.
If your son was really freaked out by this coach, then no one can make him play, regardless if you keep sending him. I would keep trying for a while longer, but be open to some other sport, art or music for him to really glow at. LD kids are usually gifted at something. Just keep trying to find something he likes and cares about. Maybe there's another sport, or even another activity that your son hasn't discovered yet.