Guinea-pig eyes are fragile, if it's gone cloudy it could've been scratched by hay (the hay is really coarse I've found) or a claw when he's been scratching.
Hay seed in the eyes usually gives a weepy eye where it waters to try and flush it out . Mine have had weepy eyes from hayseeds, usually the eye has been slightly closed and clear fluid (guinea-pigs cleaning tears are milky)
Could be cataracts
Could be dental problems (the teeth and eyes are close in the narrow skull of the piggie)
Could be early sign of diabetes
Could be something like a tumour deep seated
Could be an abcess causing pressure
Drops will mask the symptoms though - best to use a non-fluffy gauze and some boiled salted water left to cool. See if there's anything in there.
One of my sows as a child scraped her eyeball, there was always a mark and a slight white area but it didn't bother her (we had it checked, nothing could be done) She lived a good age,