Uncle Joe always gets stuck in front of the raft because he's such a powerful dude and often is the only one paddling 'cause the old ladies never do and also ends up being the person said old ladies grab onto to prevent going into the water... I'm really not a fan.... especially in July.
That being said, the last time I went rafting was in Cody, WY back in 2007 with his older brother (he was 10). The guide encouraged him to sit up on the nose of the raft which then compelled me to hold onto his life vest when we hit rapids instead of paddling (still had to deal with scared old ladies grabbing me). So I don't want to do anything crazy because this isn't my kid and I have to return him undamaged and not drown.
A full day is OK I guess, but I don't need to deal with Class V rapids while trying to keep an eye on him... I don't care about me, I've lead a full life, I also don't want his first river experience to be terrifying for him either.
In my youth, I've rafted/canoed the Delaware Water Gap many times with family and scouts. The last time, my dad and brother got sucked into a hole and got held down and had to be rescued by the ranger who was patrolling the falls, neither one of them has ever gone on a river again since... I don't want my nephew scarred.