Phone sex comes pretty naturally to me, so it's a bit difficult to advise. I usually do it by simply describing what I'd like to do to them/what I'd like them to do to me. Surely you have some sort of fantasy which you could describe for him. Really stress how you desperately need to please him, to get fucked and all the rest. Otherwise, maybe he could give you orders over the phone which you carry out while you're talking. Getting you all hot, but now allowing you to touch yourself, while you beg him for permission to do so. That gets me off every time.