By taking him or her to a vet for some antibiotics. You should take the snake to the vet anyway, especially since you got it off CL just to make sure nothing else is wrong. Boas don't often get RIs unless they have been kept in poor conditions as they tend to be more hardy in that way than BPs. But until you can get a vet appointment, you should raise the ambient air temps by a few degrees to about 90-92.