They are 4 years old, it is hot up here for the last several weeks, and it is pretty common for production to slow from the heat. I don't think it is the feed, but rather age, heat and daylight is waning.
Now he could have a hidden nest. If he was getting a bunch of eggs, and then drastically the number diminished, I would suspect a hidden nest, and last week I found a hidden nest myself in an apparently impossible place.
But if he has just been getting less and less, I think it is their age and heat. A lot of them won't lay again until late January/ February, but the eggs will be larger. However, the quality of the egg might not be that good.
Some people like to do this, all in, all the same age, all out and start over. I prefer to have a multi-generational flock - add a few each year, and cull a few each year.
Mrs K