Others have pretty well covered it, but I just want to chime in and support those saying you're dealing with a very confused and terrified dog. If there's any hint of the annoyance, frustration and anger I read in your post coming out when you're approaching this dog, he will respond to that negativity.
I completely understand if you need to have Rover take steps to place this dog with another sitter for the duration of the stay, and even if you don't, you need to report the incidents (especially if you or your dog require medical care). Your first priority has to be the safety of you, your dog, and this guest.
But whatever you do, keep in mind that a dog--no matter what they have done--is not at fault. At best, a dog has the mental capacity of a toddler; hopefully you'd have no trouble seeing how scary it could be for a toddler to be dropped off somewhere new with complete strangers and left behind. Especially if they didn't even speak the same language. Would you get angry with them for not wanting to be touched, or for hitting, kicking or biting to enforce their space? Or would you do everything in your power to make them feel safe and try to build understanding and trust?
If you decide to keep watching this dog, I recommend giving him space. Don't attempt to approach him. Don't even look at him directly. Keep your movements slow and steady and your voice low. I'd recommend sitting in the same room with him, down on the floor if at all possible, and read a book, work on your computer or watch tv. Periodically toss him treats and talk to him in a quiet, happy voice. Let him get comfortable with your presence and he may start to feel braver. Reward curiosity and exploration. Let him come to you and keep your movements slow. Don't reach over or around him. If he asks for affection, try slowly petting under his chin or down his chest for a second or two and then stop to check in. Watch his body language for signs of discomfort (freezing, turning away, lip licking, whale eye, head lowering, tail tucking, yawning, etc). If he solicits more attention, great. If not, stop. Keep offering treats for any brave and social behaviors. It may take a while, especially since he's already had multiple scary encounters with you, and pushing him too fast will backfire, but if you're able to use really good, smelly treats your progress will be faster. Given your circumstances, I'd encourage you to use the highest value treat you can (baked/boiled chicken, string cheese, hot dog, peanut butter) to make this situation safer for you and your guest.
Are you a Rover sitter? It doesn't appear that you are. Rover sitters have a link to their profile for booking purposes.
Yes I am a rover sitter.
I'm as confused at Karen is. Are you a Rover sitter with an ACTIVE account?
My guess is that "Sean" doesn't want to be identified, which is why he hasn't logged into this board with his Rover ID. I believe anyone can post to the Dog Questions board, but only people logged into Rover can post to the other two. Normally a sitter would have posted this kind of question to the Sitter-to-Sitter board, but "Sean" doesn't want anyone (especially the owner) to be able to ID him.
Just curious: Why would you not want the Owner to ID you in a situation like this?