You're more than welcome. I'd much rather an owner worry too much than too little:)
Since laryngitis is almost always secondary to something else going on, it will be important to try and determine what that "something" else might be if there's absolutely no improvement after about a week or so. Usually, this is going to require sedation so that the pharyngeal/laryngeal area can be thoroughly examined.
If a few cases, I've suggested antihistamines just on the off chance that post-nasal drip might be responsible.
Acceptable ones to use would include:
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mgtwice a day or
Claritin at a dose of 2.5-5 mg twice or
Zyrtec 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cats.
Sedation is a common side effect with these kinds of drugs
The vaccinations can help reduce the severity of disease but they don't usually prevent development of it after exposure. Vaccinated cats develop Feline Flu all the time; they just aren't as ill, in most cases.
But, he's not showing any signs..not yet anyway. If he starts to sneeze or develop a nasal/ocular discharge, then we can reliably assume his voice issues are related to an upper respiratory infection.