You are most welcome to HCM.
I have gone through your question rather your concern to your boyfriend and i think it is very much justified.
Your feelings to your boyfriend really made me touchy and i just went to the flashback with my college days when i was also dragged to the Doctor by my Girlfriend when i become seek.
Anyway, directly coming to your question ,.... "Any idea what this could be?" -
Ans: I think it to be an injury/stress
to to the vocal cord.
So, that was a very straight answer and perhaps with that answer your mind is rather filled with another dozen of questions.
I will try to explain what i meant. Injury or Stress to the vocal cord can be of many types and of course many many reasons are there. First of all we must understand why there is change of voice. True vocal cord is mainly responsible for voice production (not speech) and that is articulated by tongue/palate
and other factors to a speech/language. Whenever there is deviation of function of vocal cord from normal, it may cause change of voice. It may be weakness of voice, loss of voice, hoarseness
of voice etc..&etc... So, clearly i mean that hoarseness is only a part and other type of voice change may also be present.
Causes of stress or injury to the vocal cord again may be many but....infection, cold
, voice abuse (i.e., shouting/ excessive talking in profession like school teacher/singer), carcinoma.etc..
Now, i want to highlight one of your sentence you wrote in the question, and that is...."my boyfriend lost his voice on a trip to Vegas"...................."His voice has come back". So, voice was lost but returned in a different way i.e., hoarseness. So, definitely there is pathology to the vocal cord.
Again, please allow me to concentrate in one more line of yours...."He said it doesn't hurt at all". As far my experience goes, infective causes are painful and also recovers within 14 days (NOT 5 months like your husbands case). So, here i want to rule out infective cause as etiology.
With details you have described in your question, i assume that your boyfriend was a smoker and also not a new smoker but a chronic smoker. That is a major cause of stress to his vocal cord.
Ok..OK...i think i should stop go on explaining here because it might puzzle you more.
BUT , the question again raises..."What should you do now?"
CLEARLY, take help of an ENT surgeon. He will ask your boyfriend to go for a LARYNGOSCOPY
and might be a Chest X-Ray. You go for it. YES, i mean it. GO for it. DO NOT THINK FOR MONEY. Of course money matters a lot BUT NOT HERE. At least go for the LARYNGOSCOPY by an ENT expert. I strongly suggest you this because i think this is very important at this point of time as 5 months has already elapsed.
In the mean time, you ask him to take "STEM INHALATION", 2 to 3 times a day with "VOICE rest". Just inhale the steam coming out from hot water. No need to put any menthol in it.
In summary -
Go for LARYNGOSCOPY by an ENT expert
(1) Steam inhalation
(2) Voice rest
Finally, please allow me to take your leave now.
With these explanations i made above, i have made a sincere effort to answer your question and also taking care of your mental turmoil you must be undergoing for your loved one.
I wish your boyfriend gets well soon and you both enjoy a very healthy life ahead.
Thanks and God Bless both of you.