Hi and welcome to the BB forums! This is the right section for your question.
A rabbit should drink 50 -150 ml water per kilo body weight and day. A bun that gets fresh greens will drink considerably less than a rabbit that eats only hay and pellets, because greens consist mainly of water. If the weather is hot, the bun might drink more.
It’s generally better to serve water in a bowl than in a bottle. Clean the bowl daily with a brush or sponge under running water (you dont need to use detergent) to prevent bacterial growth which otherwise can happen in a day or so if the weather is hot. You can complement a water bowl with a water bottle and let the bun choose between them himself. Some buns like drinking from a bottle, but lapping from a bowl is a more natural way of drinking for a rabbit.
What makes you think your rabbit drinks too little? Is his appetite good? How do his poops look? Does he have good energy?
I general, rabbits that have access to fresh water dont drink too little unless they are ill, or have been ill so they’ve also eaten very little.