In some ways, I would handle it like a regular date. I would probably stop and ask if something is wrong because he does not seem engaged as you had expected. If it is a hardness issue, just explain that you had really wanted to get fucked, he was not hard and so it did not seem like it was going to happen. If he says that it will happen then keep going knowing you had put him on notice that there were expectations that the session was based on. If it progresses and still does not happen I think you have every right to adjust the fee to what services you received.