Interesting issue. I would be tempted (assuming builder is financially stable.....and I think you said he's been building for twenty years?) to leave the deck alone and let it weather.

You could readdress the issue next Spring or even next fall if you can arrive at some mutally agreeable financial condition.

I have a feeling that staining may weather out to an acceptable appearance and that might be preferable to an overcoating that may or may not hold up over time. All of those coatings look nice the first year but many have longevity problems. (usually due to application errors)