Sorry if I've got the wrong end of the stick here but do you mean you haven't seen this client before? I know you say you have but then it sounds like you haven't - it might just be me!!

I think it's important to set boundaries and make them clear at the beginning of the time together. You don't need to be shirty about it - it can be done in a gentle but assertive way. Perhaps something along the lines of, "so how would you like us to spend our time together?" would be a good opener, to see what he would like - and take the conversation from there?
If you offer a GFE then it would be unusual for him to expect 6 hours of straight banging. Also, the chances of him actually being able to do this are reasonably slim!!!

Perhaps you could think of some things you could suggest to do together - i.e. sharing a bath, massage, playing scrabble* (or maybe not!!)
*strip poker could be fun though!

Hope it works out ok for you x