I don't have experience with pool contractors, but have successfully negotiated with other contractors. Get your bids, choose the one you want to award the job, and say "I want to hire you for the project, but your quote is higher than other bids. What's the best you can offer?". Don't show your cards (ie don't tell what the other quotes are). If HE wants the work, he'll play ball.
I'll also start about 10-15% below the quote to begin negotiations. Anything lower may be perceived as lowball.