From the perspective of both a parent and an education assistant, take the opportunity (if you can get the time off work) to go on the field trip with your son. When we have students who have a recent history of behaviour problems (and normally require an EA to be with them at school), for safety and budget reasons, we can only allow the student to go if a parent accompanies them on the field trip. If it’s an extended hours or overnight trip, it costs a lot of money to pay the EA staff for all of their extra time.
A student that I currently work with is staying back from his class trip. I was heartbroken for him, but he also just hasn’t been stable enough emotionally and behaviourally to attend. Since he isn’t on the trip, though, myself and his other assistant have planned some fun activities for him in and around the community.
Now, your son may not be as unstable as this student that I work with, but it wouldn’t hurt to contact the school and find out what their concerns are about your son going on this field trip. And, if he is still unable to attend, you have every right to find out how they are going to enrich his learning experience in lieu of a field trip.
I wish you luck! I would be heartbroken if it was my own son in your position. At the beginning of the year, he punched his teacher, and was having a very difficult time getting along with others. But after some cognitive behavioural therapy and medication, he is no longer a safety concern for the school staff.