I can't provide (like giving him humiliating tasks, wants me to say things like I own him/his balls). I have no experience of being in a relationship like this. It's not proper D/s I know as there are no boundaries or anything. What should I do for the next booking?

If you don't have it in you, don't do it. it will look artificial and will be true torture for both of you. Just be honest and talk to him

I a 100% agree with @joker69 on this. You should highlight this stuff (can't do domme) in your profile so your clients won't assume and to avoid this kind of things in the future. 

Oh, I love this. I think I will definitely enjoy this if this happens to me. 

make it clear in your profile or if the guy starts asking for this in the middle of your session, tell him you don't offer this service