Perhaps he has been comparing the bid from another company. I am sure you could omit some of the procedures to reduce your price. Are you quoting $100 per hour for 8.5 hours of spreading the loam? Perhaps omit the loam...save him a lot of cost, ($1691).
So the seed you need is about 50 pounds--why does it cost $210?
Are you thinking of a mid summer seeding--bad timing--if he has irrigation it would be a slight help. Aug 15 would be much better--fall seeding. On a steep slope there could be additional problems--heavy rain--run off of soil and seed. Callbacks are costly.
You would probably save him money and get better results if you just sprayed Roundup, cut short, removed old sod and then sodded the area. Sod cost would be about 1600. Bluegrass instead of rye. If the sod company does "big roll" sod--its probably a two hour job. Perhaps you would need a slope stake in each piece of sod.
Perhaps do a soil test and add fertilizer and lime according to recommendations, (instead of loam). Use a high level of slow-release nitrogen.
MSMA for post emergent is not available any more. Post emergent crab control not needed if job is done in August--crab will not grow in the cool weather of fall, killed by frost in late October, anyway.
Get him to accept that the job is complete and done to his satisfaction and accept his check before leaving the site. Make it clear that all callbacks on on his dime.