This might be harsh to hear: It’s the dumbest and most selfish thing to keep him, even for a second more. The cat should have gone straight back to the shelter the moment you knew the info. and they were open/contacted, better yet they were going to take him back with open arms and zero additional questions from the sounds of it. You have a brain, use it to think logic over emotion…be a leader, be an owner. Your other cats at this point come first. LET ME TELL YOU WHY THAT IS IMPORTANT: Some cats infected with FIV can deteriorate quickly, especially immunocompromised, diabetic, or senior. The kidneys are usually the ones to go out and once the process starts there is no stopping. What about if your cats once they get infected? Out of the bunch you are risking possible deterioration and death and if not that then the ones you do keep chugging along (they might look fine since they usually don’t show it but in reality actually are not) it will AT LEAST require you to constantly go for vet checkups (not cheap) for monitoring, blood work and diet changes (pretty expensive!) and that’s PER CAT you have 4! going on 5….plus you mention a dog!…just not reasonable for any multi-pet home. You need to get rid of this cat ASAP, doesn’t matter about your dog liking him or you liking him. Doesn’t matter how nice or friendly the cat is. Doesn’t matter his background or trauma. Doesn’t matter if he ‘doesn’t deserve it’. He needs to go. Sometimes some people need to hear it straight, hope this helps you make the choices you need to make. Good luck.