We use the arm restraints that prevent kiddos from bending their arms in the hospital a lot, but they probably won't work for diapers. They help a lot to keep kids from reaching up towards their face and pulling tubes out. Even if the arm is straight, he'll still be able to reach into his diaper. You can try to use the baby changing table mattress if you still have it. Lay him down on it with his arms down towards his side. Take a receiving blanket or blanket big enough to wrap him and wrap around his chest/stomach area with his hands straight down near his sides. Wrap all the way around him pretty snug and then buckle the buckle on the pad. He won't be able to move his arms, at least long enough for you to change his diaper safely! We do this a lot in the hospital, as well, and it doesn't hurt them. Prayers for a speedy recovery!