As James says in his comment you need to take him to the vet ASAP.
There's various medical reasons why a cat would lose their meow (some serious some not):
Upper Respiratory-tract Infection (URI)
That's a cold to you and me, cats can get infections just like we can and just like in humans they can have a sore throat and it can lead to laryngitis and a hoarse or inaudible meow.
Vocal cord growths
These can be (and often are) benign - a non-cancerous tumor and polyps are all possibilities. Unfortunately while it is rare throat cancer is also a possibility.
This is where nerve damage has occurred that is impairing the larynx (voice box) from working correctly. It's really pretty rare in cats (dogs seem far more susceptible for some reason) but it is very serious and needs treatment ASAP! Things to watch out for:
1 Harsh breathing noises (wheezy or rattly - like an 80yr old two-pack a day man!)
2 Inspiratory stridor (high pitched creaking sounds while breathing)
4 Raspy meow
5 Dysphonia or total lack of voice
6 Dyspnea (difficulty breathing), which can lead to:
7 Pulled back lips
8 Dark or purple coloration of the tongue
9 Wide eyes
It sounds unlikely in this case - as you normally would see a significant weight loss as well but hoarseness/loss of meow is a potential symptom and if they feel it necessary the vet can test for this with a simple blood test.