It sounds like a rectal prolapse(exactly what you thought, anus flipped inside out).
This snake needs to get to a vet asap. I had a baby retic that had the same problem. The vet pushed the tissue back inside and then put a stitch on either side of the cloacha, making it smaller temporarily to keep the tissue inside.
Obviously, I couldn't feed the snake for several weeks until the stitches came out and then it was a very small meal for a few feedings to make sure the food was passing without causing the prolapse again.
Since the boa in question hasn't eaten for 4 weeks already, you need to get him in soon. Don't worry that much about the food issue. Boas can go a very long time without eating without a problem.
One other thing...it could be the hemipenes as well. Either way get him to a reptile vet.

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