When you pull the plug, he will undoubtedly try and make you feel bad. Something about letting everyone with Diabetes down, etc etc.
I think you started off on the wrong foot by purchasing the domain yourself. That’s not really a part of your services, you design and maintain websites, you don’t buy domains for people!
I would (politely) explain to him that he’s requesting more of your time than your paying clients and that (if you wish to continue) he needs to limit his requests to one day per week. Let him know that on October 22nd the domain will expire, if he’d like to continue using your services, he will need to pay for the renewal and a small hosting fee.
Was there a contract signed at all? Is the design now his property?