Have you tried feeding in his enclosure and not moving him to a separate tank?
If you are going to try offering a live rabbit or guinea pig, be very careful. Rabbits, while soft and furry, can become hostile, aggressive, and downright mean, and they will fight once they realize they are cornered.
I only suggested the vet trip because this boa hasn't eaten in 8 months and that is highly uncharacteristic of a boa...