What should I do?

So I'm doing a job for this guy running a new sewer line from his existing sewer about 30 ft to the shed out back that he's making into a apartment. He's going to have a shower, a toilet, a kitchen sink and a lav sink plumbed in with waste and water. I rode out of contract and was going to charge him $400 for the whole thing only he's taking care of the parts and I'm just charging him for my labor and knowledge. When I made out the contract and only charged him $400 I was under the assumption that he was going to help me dig half of the trench and so far he hasn't done any of the trench and I'm almost done with it. If I knew it was going to be like this I would have charged him five or six hundred. So I already made out the contract so I know I can't change it and I didn't put any stipulations on it so is there anything else I could do besides shut up and keep going?

CBB

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