Poisonous snakes have 'big cheeks' - the poison glands- which make them look like an arrow with a flared arrowhead on the end for a head. The head does not smoothly transition from the neck and is VERY obvious. Put your index finger tips together and your thumb tips together and you will create the shape of a rattlesnake or water moccasin head. If the little guy's head looks like that, then it looks like you've got a poisonous snake. That's still no big deal. If dealt with gently they tend to be laid back; they don't want to bite anything they can't swallow if they can avoid it.
If he moves sideways in a distinct 's' shape he is probably a sidewinder. While they are rattlers, they're not that toxic and are mild mannered. You really have to provoke them to get a reaction. Sidwinders and other poisonous snakes tend to be short and 'fat,' with coarse, rough looking keeled scales, not ribbonlike and sleek like a gartersnake. Older rasttlers don't rattle unless they are upset.
He still won't bother you, unless you step on it. Try to set something nearby on the floor which he can crawl under. Then lift your feet so he is exposed. The snake will probably head towards the nearest 'cover' - your object you put on the floor. At that point you can get up and go get a bucket or paper bag.
Chances are your cats brought him in and if they can do it, he's not poisonous. So let the cats handle him.