@Mita127
Your listing makes it very clear, both in the house rules and the description, that the room is for one person only and no extra guests are allowed, so I think you would be perfectly within your rights to politely remind the guest again that it's in the house rules which he agreed to when he booked and that sorry, no, you will not make any more exceptions.
If you do decide to allow it, then I think it is reasonable to ask for a fee for an extra person. You are not running a charity. Although, normally I would say you can't charge for something not mentioned on your listing, but seeing as you've clearly stated no extra guests, you need to present this as you are willing to make an exception, but only if you are reimbursed because it is not fair to expect strangers to use your home for free.
The problem is, when you make an exception to your house rules once, the guest is going to think you will make further exceptions, either to that particular rule or to others and it sounds like this guest is really NOT paying attention to your rules, so I would not encourage this. Personally, I would go the first route and politely, but firmly, tell him no. I would also ask him to have a re-read of the house rules. Sure, this could result in a bad review, but I don't think hosts should be held hostage to that, especially when you are sharing your own home.
In terms of going forward and preventing this in future, what do you do to ensure that guests read your rules and take them seriously? In my experience, most guests do not read the extended house rules or the full listing description, so you need to MAKE them do it.
I will not accept a guest until they have confirmed in the airbnb messaging system that they have read the full listing, including the extended rules and agree to them. I will also highlight a couple of key points. These should be whatever you find to be regular issues/things that guests need to understand but might have missed. For me, this is usually that I have cats and the smoking policy and, depending on the room, stairs and traffic noise.
Because a lot of guests say they have read the full rules when they actually haven't, I include an 'Easter egg' question in there that they need to answer in order to book.
This might all seem tedious or even heavy handed, but I have found it necessary, especially as I host long term guests in my own home. I'm not saying this totally prevents some guests from still breaking rules or complaining about stuff clearly mentioned on the listing, but it does reduce it significantly.
As for extra guests/visitors, I used to have a maximum of two people per room with a higher rate for two. However, like you, I now have a maximum of one person. In my rules, I say no unauthorised visitors and that visitors cannot stay overnight. However, I do allow guests to have an additional person stay for short periods if they ask in advance and there is a fee for this and that is stated in my rules too. So far, I am finding this works. I've only had one guest who seemed a bit put out about having to pay more for someone to stay over but she was a bargain hunter from the beginning. Everyone else has agreed and paid.
If that seems too complicated, then just stick to your rule about no additional guests/ visitors, but then you need to be prepared to say no the first time a guest asks.