I'm glad he's trying something new. I would encourage you to keep a positive attitude. 1 out of 2 is a great success rate. Maybe there would be another option if the history class is not a good fit. It is fantastic that he tried to use pencil and paper to do his writing. This is a helpful skill in life and it is good he's able to improve on it.
My suggestion would be to try to give him more space. Especially since he's older and wants his independent experiences, it really takes away from that if you are in the classroom. Also, I think it could really amplify his anxiety because it gives you an opportunity to worry over every detail instead of just letting him work it out. It really shouldn't be your job at this point to worry whether he misspells a word or uses the wrong punctuation. He will be fine without you there. There should be many volunteer positions at the co-op. I would explain that your son is older and is working on being more independent and you don't want to be in his classroom. You could work in another classroom or do another job.
One value of independent experiences like co-ops is that it can be easy at home to get in a rut where you don't expect certain things from your kid. Different approaches can give a kid the opportunity to surprise us - as he did with the amount of writing which is more than you thought he could do.
