This sounds nutty, but it works. Have a water can full of water ready. Each time he pees, immediately soak that spot with no less than 1 pint (2 cups) of water. This dilutes the minerals which are too strong for the grass, and you avoid the yellow spots.

I know someone who just keeps a small pot with water (picked it up at a yard sale) it holds the right amount of water, and she fills it each time, leaves it by the back door ready for the next trip. With 4, I need MORE than one pot of water, lol. Anyway, each time out, she picks it up as she goes by, uses it, refills it from the hose, sets it back in place for the next trip. (Yes, she is VERY organized. )