We had a five year old boy on our trip last summer. He had 2 older brothers travelling with him. He had a blast, but he did not do the Delicate Arch hike. I don't remember if he went white water rafting or not--the rapids were very calm when we went (only a class 1/2--and that was mainly for the rocks underneath the waves, where a less experienced rafter could get stuck).
I agree with JenBadeaux--it depends on your child, but most of the activities are certainly manageable. The hardest thing would probably be the bus trip from Grand Canyon to Moab, which is a long day. As long as he could handle amusing himself on the bus and has pretty good stamina--he should do ok. (But there is really no time to nap during the day--except on the bus.)
Good luck!
Turtlemom