If your cousin wants the best synthetic stock, then he must be willing to pay for the best. I personnally, like H-S Precision and McMillan, but these stocks ain't cheap, either in quality or price.

For mid-price range stocks, I would look at Six Enterprise, High Tech, or Hogue. If he wants a synthetic stock for the sake of having a synthetic stock, then B&C, Ram-line, or Butler Creek will do.