If your hair loss is hereditary, for instance, you might have more difficulty in regrowing hair, but that doesn't mean it can't be done, and you may be a good candidate for zinc supplementation. (hairlossrevolution.com)
If he is not really itchy, you may want to have your veterinarian check his thyroid - hypothyroidism is a relatively common reason for hair to not regrow after shaving and it's usually easy to treat. (1800petmeds.com)
Hair usually regrows in 2 - 3 mo, and remission may last from 6 mo to 3 yr. (lowchensaustralia.com)