He could just be getting older and hoarser. But difficulty crowing can be a sign of a respiratory disease, such as Infectious Bronchitis. If you want to be safe, I'd get some Oxytetracycline (sold under names such as Terramycin, Duramycin, Tetroxy HCA-280, etc.), and treat him and perhaps the rest of your flock. The Oxytetracycline can generally be found at a livestock supply store, for a rather minimal price ($9-$15).
The Oxytetracycline dosage is 1/2 teaspoon powdered Oxytetracycline per quart of drinking water for 7-14 days. During treatment, do not give any probiotics, dairy products, or Apple Cider Vinegar in the water. Vitamins/electrolytes are fine, though. Now, if your rooster had clear signs of a respiratory disease, I would recommend treating with Tylan, as Oxytetracycline isn't quite as effective as a stronger (but more expensive) antibiotic. However, since your rooster appears fine, except for the trouble crowing, I think that treating with Oxytetracycline is a good preventative measure.
By the way, he is a really beautiful rooster!
