Hi,
Just to try and rule other possible causes out;
Has the pet store been treating for mites or anything on or near the feeders?
What kind of bedding do you keep the sick snake on and have you changed it lately?
Have you double and triple checked the temps in the enclosure with a reliable digital thermometer in the place the snake actually lies ( i.e. inside the hides)?
Also, if he was keeping the boa at the pet store then the fact it was previously kept with BP's means very little as it has just been possibly exposed to a whole pet stores worth of parasites and diseases. Did the pet store get any new animals in lately and are any of them ill? I ask because that also makes the health of their baby BP's and burmese at the time of limited use - but how they are now is still important so I would ask.
It might be possible to get the vet near you to go read up on the various parasites and diseases that can lead to similar symptoms - also getting a parasite check done might be wise. If the worst happens and you lose the sick one I would definately think it would be worth the money to get a necropsy done to see if you need to take any steps to protect everything else.
Best wishes.
dr del