It may be that you have a fussy guinea pig, or that he may have trouble drinking from the bottle, or is un-familiar with it. This is perfectly normal. Instead of using just a bottle I suggest you fill a bowl with water and clean and re-fill it each day as well. This way your guinea pig will always be able to access water. This is actually how we give our guinea pigs their water here!
Over time you may be able to teach your guinea pig to drink out of the bottle, by dipping your fingers in the water and holding it next to the bottle. He may get the idea on what the water bottle is there for!
I would also try to put the bottle up to their mouth and use your finger like a little guinea pig tongue to get some water drops coming out. They might understand if you do that, then offer them a drink. Try this a couple times.
If they don’t seem to drink out of the bottle right away (maybe a day or so) then I would put the water bowl right under the spout of the water bottle (with no water in the bowl) If they still don’t drink any water then I would put water in the bowl, but leave the full water bottle hanging right above it!
Also if you give your guinea pigs veggies everyday, then they are getting some water. Try rinsing the veggies but don’t shake off all of the water, this way they can get some extra water off their veggies!
I hope this helps.
All the best,