If he is the only cockerel around and she is both healthy and in lay, he is likely already breeding her. At this time of year, many birds are moulting; a moulting hen is not laying and therefore unreceptive to a rooster's charms.
I have a 6yo hen whos still fertile, but at 5 years old she could be pressing her luck. How old is the rooster they can become infertile sooner than hens. Have you tested him over another hen to insure he is fertile? You can aways artificially inseminate her with his sperm.
Also I never run more than 8 hens over 1 rooster to insure fertility. 1 rooster over 20 hens is alot to cover lol.