Basically you have two choices..if you are desperate to keep his business you will have to go through with it,but then what do you do next time??..if you are not then i would be polite and apologetic,saying i have thought it over and decided I really need to keep my business head on and just see you for the time  booked.i cant afford to stay overnight if it isnt an agreed booking.Sorry about saying i would in the heat of the moment,but it just isnt feasible for me in reality.
You will probably lose him i admit.But its a lesson learnt about being assertive at all times and dealing with boundaries.

Thanks ladies.

So I text him half an hour ago after reading your replies. This is what I put:

Hello, sorry to spring this on you last minute but upon thinking about tonight I've decided I can't stay overnight. I'm not comfortable with overnight bookings so I apologise for agreeing to stay over before but I felt a little on the spot and didn't want to sour the meeting. I'm happy to come for our usual 4 hours but I will be going home afterwards as staying over until tomorrow isn't feasible for me.

He's text back that he understands and still wants to see me tonight but he's cancelled our other meeting which was due to take place at the end of the month. On call at work apparently. He said he was fighting it because he knew he was seeing me but it looks like he may be on call after all.

That makes me think he's only let me down on that appointment because I'm not staying the night, so he obviously assumed I would be even though no discussions took place about me staying the night on that occasion. I'm a little annoyed by it but I guess he's annoyed too because he thought he would have me there the entire night tonight.

 Part of me thinks he would probably cancel today if he was able to get his money back on the hotel lol. Is there a time scale which you can get a refund on hotels, I'm not familiar? He just seemed very quick to cancel our next appointment at the end of the month but not so quick to cancel tonight's (it has been booked for some weeks)

Oh well lesson learned!

I wouldn't be surprised if he's bluffing to try an get you to commit to an overnight. I'd bet that even after he sees you won't play ball he'll "suddenly be available" for those bookings afterall. 

Even if you do loose him as a regular (and a 4hr regular is a good one), he sounds like a boundary pusher, I've had my fair share
of them in my time, thinking themselves special and only booking for an hour but staying for two or three as a "social" visit and we
only spent one hour in the bedroom. (His reasoning, not mine obviously..!!)
He sounds like one of those (I could be totally wrong), but they'll try and get away with it if you let them!
And like Kay said, if it doesn't feel right, it usually isn't!

Even if you do loose him as a regular (and a 4hr regular is a good one), he sounds like a boundary pusher, I've had my fair share
of them in my time, thinking themselves special and only booking for an hour but staying for two or three as a "social" visit and we
only spent one hour in the bedroom. (His reasoning, not mine obviously..!!)
He sounds like one of those (I could be totally wrong), but they'll try and get away with it if you let them!
And like Kay said, if it doesn't feel right, it usually isn't!

I've come across this phenomena too, often after agreeing to stick at reduced or regular rates and only leaving when I put a foot down. Thank goodness for clients who do not even dream of being like that, but it can be draining.

Personally I don't like going back on my word, so for me it's about making it clear once I've fulfilled whatever commitment I've made that it won't be happening again ie would have been before he'd rebooked.

I hope it works out for you.