I can see why your husband is feeling more and more threatened by this. One 'exclusive' relationship is different from the string of men who pay, fuck, and then fuck off, plus you've been prepared to have surgery to please this client via money that your husband either couldn't or didn't want to help pay for.
If you can do so honestly, you might consider letting your husband know that if he says he wants you to end the client relationship, you will do so without kicking up a big fuss. Knowing that you would do this if asked may well mean that he never does ask.
If you can't do this, then I wonder how long your marriage will last because the feeling that you are now the client's mistress, and more and more dependant on him, will grow. The more you can do to not rub this in your husband's face, the better - so think carefully about agreeing to long holidays or other expensive treats with the client.
On the client side, you need to work out what you're prepared to do and for how much. Are you prepared to drop everything and go over to him straight away if he calls, for example? (If so, it'll be a sign to your husband about who is really the most important in your life.) How and when is the agreement to be renegotiated? What do you know about the client's life / sex with other people?
Oh, and it's not unknown for escorts to agree to such a deal and then continue with other clients. This is one reason why sugar daddies can get so demanding at short notice - it's a way of disrupting all your other arrangements.