Your son can still go but he would possibly need to ride in a stroller, wheelchair or
ECV depending on age. I have a son with mild CP and we have always been able to do fine without a GAC for him but we are lucky to have a few school breaks at very slow times so in the morning hours we can do plenty with some ice cream breaks and then we rest for a few hours in the afternoon. It's different but it may turn out OK. They've always encouraged people with endurance issues to ride rather than get a a GAC because besides the lines there is so much other walking.