Guinea-pigs are experts at hiding things.
Your piggie is 'middle aged' and like middle aged men are prone to things like (for example) diabetes (which causes thirst) , kidney or bladder stones, over heating f he's indoors and the room is warmer.
Have you changed any of his pellets or hay brand, might it taste different or be more salty.
Does he normally have more veg (mine are eating more root veg and celery type foods now. All day eating grass has been replaced by hay/Readigrass)
Could just be boredom , he's noticed the water bottle and decided to have a go at it more.
Is he peeing easily? No hunching or squealing?
No blood in his wee?
If you look at a dry patch is it more chalky ?
If you take him to the vet they can test his pee and do an XRAY For stones if there are symptoms.
They are complex little animals , hide their illnesses and as they get older they get more vunerable and the risks of having any type of sedation for tests increases.
I think I'd try moving him from any obvious or potential heat (don;t let him get cold though 
) maybe cut down on dark greens and keep a close eye.
Are you sure the water is being drunk , not a leaky bottle?
Does he have a cagemate?
Is he neutered?
We've had two un-neutered boars , one had the most amazing bladder (I'm sure he was part camel, he could hold loads then pee a river)
Two neutered ( not hampered by the mahoosive guinea-pig 'nads) . Our GP6 never drinks but does pee pretty well , he's not a 'marker' like GP1 was .