Patti, did you check his vent area to make sure it is clean and not obstructed with a ball of manure or excess feathers? Sometimes it helps to take a scissor and trim away those feathers above and below the vent. Check for signs of mites at the same time, as a heavy infestation will drain them. And I would try taking him away from all females for a period of about two weeks.
But Royce is right in that low vitality with no apparent causes may well be inheritable.
I'm not saying any of these are problems with the cock in question, but a couple of Mike's points reminds me of an unwritten check list I "try" to follow each year before collecting hatching eggs.
About a month before collection eggs to incubate setup breeding coops and flocks, feed them a good breeder ration with no supplements that will dilute the nutrients and keep unfrozen water in front of them 24/7.
A female chicken will only lay so many eggs in her lifetime. She can't lay more than what she started with and once they are laid that's it. The eggs she starts with develop and many to most will get laid during her lifetime, but she doesn't produce any new eggs. Under normal conditions (no added artificial lighting) she may lay for several years, but if she is subjected to longer days by artificial lighting she will lay her quota of eggs sooner.
Knowing that my hens can only produce a limited number of eggs I supplement sunlight with artificial lighting during the hatching season to get the most eggs when I want them. During this month prior to collecting hatching eggs I start the "warm" spectrum lighting for 12 hours per day and increase it to 16 hours by the time I start collecting. Of the visible light spectrum "warm" or red light about 2700K to 3000K is the most beneficial to increase egg production and by using LED blubs I save on electricity. Don't leave lights on continuously, since that can stress the birds.
This is also the time to trim feathers around the vents on both males and females. I don't do it because there is necessarily too much fluff, but because I want them to have every advantage possible to produce fertile eggs that produce chicks. As Mike said, sometimes manure will build up on the feathers in this area could obstruct mating.
Also, each breeder should have a few drops of Frontline applied, depending on their size.