If we told our spca he was agressive they would put him down righ on the spot. We want to at least give him a chance at being adopted out.
What if he gets adopted by inexperienced dog owners with a cat? And he kills their cat? Then they would probably put him down or take him back to the shelter... where he would go through the same cycle again until it was too late for him

Its best to tell the shelter absolutely EVERYTHING.