I would take the boa to the vet and have them examine the stomach, well crap, it's the weekend- unless you are willing to spend money to see a weekend exotic doctor, that's going to be difficult.
There is some hope that maybe when the boa entered the ball escaped the same way, look very very hard. Everywhere. I searched for 6 hours straight before I found a baby ball.
Or it is possible the boa did get him. I don't know, but I wouldn't risk not looking. Can't give up if you don't know for sure. I wonder how he got into the ball's enclosure?