I hope it's allergy related, too.
Also, smoking is known to cause repeated bouts of laryngitis, because those who smoke are more prone to throat dryness, which is a major contributing factor of laryngitis. So, it will be beneficial for your husband to quite smoking.
Coughing and post-nasal drip may also contribute to laryngitis after a person has been exposed to allergens or irritants.
Is your husband suffering from chronic sinusitis?