I would probably laugh a bit too - you know, to try to keep things lighthearted! - and say, "I know, isn't it wonderful?? You don't have to deal with my boring complaining or needy whining!" (Maybe don't say 'needy whining' since that might be his plan, ahem.)
To be honest, I feel that declaring, "You're only nice because I'm paying!" in a plaintive way to a prossie is like going into Waterstones and shrieking, "YOU ONLY SELL BOOKS AND OTHER ENTERTAINMENT ITEMS!" ...i.e. totally pointless. I mean, the vast majority of clients understand perfectly well what they're paying for, and we enjoy giving them their service, and things all go swimmingly. It's only very selfish or immature people who expect things from others that they are clearly unable to provide and while my heart does ache a bit for the men who find buying sex to be a confusing and upsetting process, the fact is, if they don't enjoy sex unless the woman is there of her own volition, he ought not be seeking out prostitutes. Let alone seeking out prossies and trying to emotionally manipulate them into feeling bad for him so that he can maybe get a pity shag or even just a pity hug, depending on just how desperate he is for something 'real'.
Basically, "maintaining boundaries" is almost impossible with a client who's clearly not good with boundaries in the first place. It's like how you can't really turn a timewaster into a decent client no matter what you do so best just sharply reduce the amount of time you spend dealing with them because, as business-owners, we need to pay lots of attention to the great customers and not get burnt out by the bad ones. If a client tries to turn a booking into a free date, or tries to manipulate me into a 'relationship', or seems otherwise unable to understand and accept my boundaries, I will deal with him kindly but firmly because I'd rather not string him along to try to turn him into a regular. He's always going to be confused and difficult to handle, so best say goodbye before things get really nasty and he decides A) he 'loves' you, so B) you are not allowed to see any other men, so C) he's going to start following you around and making threats.
Sorry. I know that probably seems super-strict. But escorts are already a vulnerable group when it comes to stalkers (and worse) so I think that when you see the first, mildest signs that someone struggles to understand the nature of the transaction, that's when you politely and quickly remove them from your working life.