FWIW, I tend to agree with those who say 'work out a 3-5 yr deal'...explain it to them in terms of discount, show them that the other company cannot possibly guarantee 'life', and that if you signed such a deal it would expose them to a huge risk, because they would have no way of controlling the quality of their SEO provider...for all they know, you or your comp could just take the $, work on the site for a year, then forget about it or go out of business.
The time value of money equation tells us that $7k in your pocket now is worth a whole lot more than that same $7k paid out over 3-5yrs...so yeah, it makes sense to go even 5yrs if it is an easy site to rank...sell it to the dentist that they are not only getting a HUGE discount by signing this deal with you, but that they are locking in TODAY'S prices. The guy is a professional, and you want to show him that you're taking this seriously, protecting his welfare, and offering him a great deal at the same time.
The point is to take that $7k and scale YOUR business now...you can do a lot more with $7k today than you can with $300/m spread out over years. Nonetheless, don't be afraid to walk away...and make sure he knows that you are doing it for moral principles...he'll respect you for being a pro rather than a hustler.