Cut your losses now, because the end will be even more expensive.

Not all designers are equal. I agree with sushipup1, cut your losses now.

If you discussed your budget with him and he came in over your budget by 78% then he didn't listen to you at all. Unless he has a plan for a phased in design....

ETA: If he couldn't do the job for your budget he should have disclosed it upfront before the $500 design fee. He knows if $15k is doable or not.

I'm glad to hear this. Did you get his plan? My bet is you can copy a lot of the bones of the plan without his help. Then, you can hire someone to help with some of the remainder items. But, that's only if you like his plan.