If he was a drop off, NEGOTIATE the price to near nothing. The petstore has little to nothing invested, point out he is inj poor shape and will require lots of TLC to return to good health. Maybe take some pics of your healthy BRB's with you as an example?
Not uncommon for a BRB to be off feed when dehydrated. Work on the water first, large surface area water dish will help. Close off the ventilation to almost nothing. Misting will help if the water is trapped. Make it very damp and clean OFTEN. If the door has condensation....WOOHOO, humidity is very high.
I sold a baby to a petstore and told them clearly what they needed to set up for him. They ignored the breeder that rasies hundreds and put him in a screen topped 10 gallon with heat. DUH, he dried out and wouldn't eat for them. When I pointed it our to them the reply was we mist him twice a day. My immediate reply was yeah and it rains in the desert sometimes but that doesn't make it humid. Long story short, breought him home, rehydrated him and wow, he started eating again.
I'm sure you probably already know most if not all of this. More for the lurkers, you know who you are 
Good luck with this and keep us posted, like in my previous note, My prize BRB was a rescue. And have LOTS to compare her to.
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Thanks,
Dave "Rainbows-R-Us"
0.1 Wife (WC)
0.2 kids (CBB)
4.12 Brazilian Rainbow Boa
2.1 Hypomelanistic BRB
0.1 Het for Hypomelanistic BRB
0.1 BCI "Elvira" normal from 1989
1.0 BCI albino / het-anery
0.1 BCI Hypo / het-albino
0.1 BCI Anery / het-albino
0.1 BCI Hypo (possible super)
1.0 BCI albino het stripe
1.0 BCI salmon hypo
0.1 BCI ghost
lots.lots.lots feeder mice and rats 