If you really like him and feel you would want to spend time with him without payment, then do what I did: "promote" him to boyfriend status. I've been in this arrangement with my BF since about July last year. I'd met him twice as a client (including a 4hr dinner date) while I was working in London. He texted me to say he was coming through my home town on the way back from an event and did I fancy a drink. That was the point at which I decided "I want this guy as a fuck buddy" so I said yep let's meet. The rest is history
For me it's nice and easy - he's married but they both play with others. I see him about every 1-2 weeks. We are fond of each other and message daily on Skype, but there's no danger of a real emotional attachment.
I've been there, done that, and don't envisage ever being in a close LTR again. I'm far too independent.
If you are younger and you feel like you are going to want a committed LTR at some point, then you need to think carefully about whether you want that to be with a client, though. The advantage with a client is he already knows what you do and if he has a problem with it then he's a massive hypocrite. The disadvantage is going to be if you want a monogamous relationship, he may not be the sort.