coaching in sport (any ideas?)

My son is 11 and has a diagnosis of high functioning ASD,  he currently does archery, my Husband and I shoot as well.   when he is doing well he is fine and last week after not letting him do any competitions for a while we let him do one.    He did amazingly well and shot with a grown up who was amazing with him and he really clicked with,  we shot at the same time so he didnt have a lot to do with us

roll on today he shot the morning session we were watching not shooting and he was a total mess,  as soon as he got a bad shot he turns round and looks at us and asks what he did wrong , which is a no no in archery,  then after he shot he gets louder asking what he has done wrong and crying,  on the line at one point he was stamping,  if we try to give advice he takes it so personally as if we think he is the worst person in the world.   It got to the point that we deemed he was maybe putting people off even though they never said and were trying to be encouraging with him.

He wont give the sport up as we go and he still comes and his brother gave up and he refuses to be the same as his brother.    He expects to do well every week but archery is an up and down sport and very technical and trying to get this trough to him is really hard.  we tell him to relax and not worry about his score,  we explain all the things he should be doing without being critical but it becomes quite stressfull at times.   

my question is has anyone ever coached autistic children and how did you deal with the negativity.   He loves the sport when he is doing well and we really want to help him continue without the stress as any talk about quitting is sometimes even more stressfull.

thanks