The problem is less with him losing his benefits, and more that you could be held responsible for allowing him to misspend funds. He should provide you with all the bills so you can see what he has spent the money on.

I would suggest he either needs to close the CC account or agree to a fixed amount that he can charge per month. Otherwise, you're allowing him to circumvent your role as Rep Payee.

If he refuses, then I would advise you make him use a Rep Payee service. They'll charge him 10% of his benefits, but it'll protect you.

It doesn't matter how you pay the bill, but again you shouldn't be blindly paying it.