I'd consider not going if you would be doing something like driving your son in because normally he gets a bus which can be unreliable. You may be aware that if he's very late to an exam, he won't be allowed to sit it (JCQ rules).
If he can walk to school, get reliable public transport that will get him there early with a back up option if he misses it, or your DH/his sister can drop him in, then I think it's fine to go.
If his only option was a school bus, where if he missed it or there was a problem with the bus, there is no other option to get into school on time AND you are the only person who could drop him to school on those days, I'd say he needs to take priority.
In terms of emotional support, etc, I think it's possible to do this over the phone if he wants it, and he will have his teachers and friends in school, as well as DH and his sister at home- so I think it's fine for a few days.
I think the only exception for this is if you don't normally travel for work, and this would be the first time- because he might find that a bit emotionally unsettling, and I wouldn't want to do that to someone taking exams.