"Properly priced" real estate is currently selling within 3-6% of it's list price in this market area (bank owned property can push that further). Therefore, 15% under might be considered a low-ball to this builder. If it's been on the market for as long as you say, he's clearly overpriced and the market has shown him that. Your job (or better yet, your qualifed Buyer's agent's job) is to exemplify his error with valid sales comparatives that support your price position.
If he still won't bite because he "needs" more you can point out that the market is expected to continue to decline ..... and that the market doesn't really care what anybody needs.
My advice ... Get a qualified buyer's agent to assist you with the negotiations and comps .... it's what we do!
Good luck, Andy