Are you 100% sure that he isn't drinking his water? Because budgies will only drink their water in front of you if they are fully comfortable around you. Like my budgie Mickey, who is very outgoing, he will drink water in front of anyone whereas Chicky absolutely will not. She will literally go to the water bowl and look around to check if anyone is watching. I guess it makes sense since Mickey has been with me longer. It's something about the fact that they are in a vulnerable position when drinking water and that they won't be prepared if a predator attacks. Also, you won't see the water level change. They don't drink much. Meaning they drink regularly but the amount is really small. Like when Mickey is thirsty and can't find the water bowl for some reason, he'll fly to the kitchen and move from shoulder to shoulder and make such a big deal out of it but when we actually turn on the water, he'll drink a drop or two and fly away. They're small birds so they eat and drink in small quantities. I haven't seen Chicky drink for over a week but she is just as happy and her poops are completely normal. So you probably aren't noticing when he's drinking water. You can't monitor him 24/7 and he's probably just drinking in his cage. So unless you see any difference in his poops or his health, I wouldn't worry too much. If you want, set a little camera in his cage and you'll see him drink his water. And I don't know what you're using, but I've heard that stainless steel water bowls are the best because they don't grow as much bacteria.